Acoustic condition sensor employing a plurality of mutually non-orthogonal waves

ABSTRACT

A touch sensor, having an acoustic wave transmissive medium having a surface and a touch sensitive portion of the surface, a transducer system for emitting acoustic energy into the medium, and a receiver system for receiving the acoustic energy from the substrate as at least two distinct sets of waves, a portion of each of which overlap temporally at the receiver system or overlap physically by propagating in the touch sensitive portion along axes which are substantially non-orthogonal, the receiver system determining a position or a waveform perturbing characteristic of a touch on the touch sensitive portion. The receiver may be an amplitude detector or be sensitive to a phase-amplitude characteristic of the received acoustic waves.

This application is a divisional of U.S. patent application Ser. No.08/695,716, filed Aug. 12, 1996, now U.S. Pat. No. 5,854,450, which is aContinuation-in-Part of U.S. patent application Ser. No. 08/424,216,filed Apr. 19, 1995, now U.S. Pat. No. 5,591,945, each of which isexpressly incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an acoustic touch position sensor, andmore particularly to such a sensor wherein a coordinate position, andoptionally an absorption characteristic, of an acoustic disturbance isdetermined by analyzing a plurality of received signals. The presentinvention allows the sensing system to employ waves to differ in pathgeometry, and/or wave characteristic type. e.g., mode, frequency,waveform, velocity, and/or wavelength. This system advantageously allowsredundant position measurement and/or differential wave perturbationsensing.

BACKGROUND OF THE INVENTION

Acoustic touch position sensors are well known. A common system includestwo sets of transducers, each set having a different axis alignedrespectively with the axes of a physical Cartesian coordinate systemdefined by a substrate. An acoustic pulse is generated by onetransducer, propagating as a Rayleigh wave along an axis whichintersects an array of reflective elements, each element angled at 45°and spaced corresponding to an integral number of wavelengths of theacoustic wave pulse. Each reflective element reflects a portion of thewave along a path perpendicular to the axis, across an active region ofthe substrate, to an opposing array and transducer which is a mirrorimage of the first array and transducer. The transducer in the mirrorimage array receives an acoustic wave consisting of superposed portionsof the wave reflected by the reflective elements of both arrays,directed antiparallel to the emitted pulse. Wavepaths in the activeregion of the sensor have characteristic time delays, and therefore awavepath or wavepaths attenuated by an object touching the active regionmay be identified by determining a timing of an attenuation in thecomposite returning waveform. A second set of arrays and transducers areprovided at right angles to the first, and operate similarly. Since theaxis of a transducer corresponds to a physical coordinate axis of thesubstrate, the timing of an attenuation in the returning wave isindicative of a Cartesian coordinate of a position on the substrate, andthe coordinates are determined sequentially to determine the twodimensional Cartesian coordinate position of the attenuating object.

The applicability of such systems as commonly employed is restricted bythe following major limitations. First, acoustically absorptivecontamination in localized regions, e.g. a water drop on a knownRayleigh-wave sensor, result in large areas of shadowing in whichtwo-dimensional touch positions cannot be reconstructed. Second, theconfigurational requirements of these sensors limits their versatilitywith regard to shape and size. Third, reconstruction of touchcoordinates may lead to ambiguities when more than one touch is appliedsimultaneously. Finally, such sensors provide limited touchcharacteristic information from which to differentiate valid touchesfrom false touches, e.g. fingers from water drops. The present inventionaddresses these problems.

Present commercial touch screen products generally serve applications inwhich the touchscreen is an input device that is intended to be used byone user at a time. An automatic-teller-machine (ATM) bankingapplication is typical. While many customers may sequentially use atouchscreen based automatic teller machine, each user in turn has aprivate dialog with the system. In contrast, few if any touchscreenproducts are presently available for applications in which thetouchscreen is an input device that is intended to be used by more thanone user simultaneously.

a. Parallel Transducer Arrays

Acoustic touch position sensors are known to include a touch panel orplate having an array of transmitters positioned along a first edge of asubstrate for simultaneously generating parallel surface acoustic wavesthat directionally propagate through the panel to a corresponding arrayof detectors positioned opposite the first array on a second edge of thesubstrate. Another pair of transducer arrays is provided at right anglesto the first set. Touching the panel at a point causes an attenuation ofthe waves passing through the point of touch, thus allowinginterpretation of an output from the two sets of transducer arrays toindicate the coordinates of the touch. This type of acoustic touchposition sensor is shown in U.S. Pat. No. 3,673,327 and WO 94/02911,Toda, incorporated herein by reference. By employing a direct acousticpath from a transmitting transducer to a corresponding receivingtransducer, an acoustic path length which is approximately equal to theheight or width of the substrate is provided, as shown in FIG. 1.Because the acoustic wave diverges, a portion of a wave emitted from onetransmitting transducer will be incident on a set of receivingtransducers, as shown in FIG. 2.

b. Reflective Arrays

In order to reduce the number of transducers required for an acoustictouchscreen, Adler, Re. 33,151, and 4,700,176, provide a reflectivearray for reflecting portions of an acoustic wave along incrementallyvarying paths. Therefore, if two such arrays are disposed opposite oneanother, as shown in FIG. 4, a single transmit and receive transducerwill allow touch sensing along one axis of the substrate, with a maximumacoustic path length of twice the height plus width or twice the widthplus height of the touch sensitive area. The maximum acoustic pathlength is a useful metric for acoustic touch sensors because mostmaterials, e.g., glass, have a relatively constant acoustic power lossexpressed in dB per unit length; the greater the path length, thegreater the attenuation. In many cases, it is this attenuation of theacoustic signal which limits the design of the touchscreen, andtherefore it is generally desired to have high acoustic efficiency ineach of the touchscreen components to allow design leeway. Thus, forexample, greater numbers of transducers may be selectively deployed toallow larger substrates, and likewise, with limited size substrates,acoustic paths may be folded to reduce a required number of transducers.

In order to provide a set of surface acoustic waves which propagateacross a broad region of the substrate in parallel, an acousticallyreflective grating having elements set at 45° to the axis of the beam isdisposed along its path, each element reflecting portions of the wave atright angles to the axis of propagation. The acoustic waves are thencollected, while maintaining the time dispersion information whichcharacterizes the axial position from which an attenuated waveoriginated. The position of a touch in the active area is thusdetermined by, e.g., providing another reflective grating opposite thefirst, which directs the surface acoustic waves as a superposed wave toanother transducer along an antiparallel path, recording the time ofarrival and amplitude of a wave pattern, an attenuation of whichcorresponds to a touch and a characteristic time corresponding to aposition along the axis of the arrays. The touch, in this case, mayinclude a finger or stylus pressing against the surface directly orindirectly through a cover sheet. See, e.g., U.S. Pat. No. 5,451,723. Inaddition, if the emitted wave diverges, one of the reflective arrays maybe eliminated, as shown in FIG. 3, although a rectangular coordinatesystem is not provided. In the case shown in FIG. 3, the maximum pathlength is approximately the height plus the width. Acoustic touchposition sensors are also known wherein a single transducer per axis isprovided for emitting a surface acoustic wave, as shown in FIG. 5. Inthis case, the maximum path length is two times the sum of the heightplus width.

The known reflective arrays are generally formed of a glass frit whichis silk-screened onto a soda-lime glass sheet formed by a float process,and cured in an oven to form a chevron pattern of raised glassinterruptions. These interruptions typically have heights or depths oforder 1% of the acoustic wavelength, and therefore only partiallyreflect the acoustic energy.

Thus, with waves having surface energy, the reflecting arrays may beformed on the surface, and where wave energy is present on both sides ofthe substrate, these reflecting arrays may be formed on one or bothsides of the substrate. Because the touch sensor is generally placed infront of a display device, and because the reflective array is generallyoptically visible, the reflective arrays are generally placed at theperiphery of the substrate, outside of the active sensing area, and arehidden and protected under a bezel. The reflective elements of thereflective array each generally reflect of order 1% of the surfaceacoustic wave power, dissipating a small amount and allowing theremainder to pass along the axis of the array. Thus, array elementscloser to the transmitting transducer will be subject to greaterincident acoustic energy and will therefore reflect a greater amount ofacoustic power. In order to provide equalized acoustic power at thereceiving transducer, the spacing of the reflective elements may bedecreased with increasing distance from the transmitting transducer, orthe acoustic reflectivity of the reflective elements may be altered,allowing increased reflectivity with increasing distance from thetransmitting transducer.

Adler, U.S. Re. 33,151, relates to a touch-sensitive system fordetermining a position of a touch along an axis on a surface. A surfaceacoustic wave generator is coupled to a sheet-like substrate to generatea burst of waves, which are deflected into an active region of thesystem by an array of wave redirecting gratings. According to adisclosed example, surface acoustic waves traversing the active regionare, in turn, redirected along an axis by gratings to a receivingtransducer. A location of touch is determined by analyzing a selectiveattenuation of the received waveform in the time domain, eachcharacteristic delay corresponding to a locus on the surface. Theredirecting gratings are oriented at 45° to the axis of propagation, andspaced at integral multiples of the surface acoustic wave wavelength,with dropped elements to produce an approximately constant surfaceacoustic wave power density over the active area. The spacing betweengrates decreases with increasing distance along the axis of propagationfrom the transducer, with a minimum spacing of at least one wavelengthof the transmitted wave. U.S. Pat. No. 5,329,070, U.S. Pat. No.5,260,521, U.S. Pat. No. 5,234,148, U.S. Pat. No. 5,177,327, U.S. Pat.No. 5,162,618 and U.S. Pat. No. 5,072,427 propose specific examples oftypes of surface acoustic waves that may be used in the acoustic sensorsystem taught in the Adler patents.

Where a separate reflective array is provided to redirect acoustic wavestoward the receiving transducer, these are also provided with anincreasing acoustic reflectivity with increasing distance from thereceiving transducer. This is to reduce signal loss with propagation ofthe signal toward the receiving transducer along the axis of thereflective array. Typically, array pairs are designed as mirror imagesof one another.

U.S. Pat. No. 4,642,423, to Adler, incorporated herein by reference,addresses pseudo-planarization techniques for rectangular touchscreensurfaces formed by small solid angle sections of a sphere. According toAdler, reflective elements are angled to excite waves along sections ofgreat circles of the spherical surface which extrapolate to a commonintersection point. This patent addresses the need for touchscreens thatmatch the curvature of CRT faceplates, for which the radius of curvatureis always large compared to the diagonal dimension of the faceplate.This patent teaches means to minimize the inherent differences betweenspherical geometry of a small portion of a sphere and the Cartesianplane, allowing use in conjunction with controllers that are designedfor flat sensor geometry. The acoustic waves generated by the system ofAdler are substantially orthogonal. Known embodiments of the Adlertechnology include 19 inch diagonal CRTs with a radius of curvature of32 inches and 13 or 14 inch diagonal CRTs with a radius of curvature of22.6 inches.

c. Two Dimensional Position Sensing

In order to receive information determinative of the coordinates of atouch, two acoustic waves, each propagating across the active region ofthe substrate along perpendicular axes are provided. Thus, the two axesare typically used in conjunction to recognize a valid touch, but mayalso be analyzed separately and non-interactively to sequentiallydetermine a position along each of the two orthogonal coordinate axes.In these known systems, the coordinate axes of interest to theapplication are defined by the physical configuration of the sensor.Thus, sensor design is constrained by the requirements of theapplication's coordinate system.

In known systems, the system operates on the principle that a touch onthe surface attenuates surface acoustic waves having a power density atthe surface. An attenuation of a wave traveling across the substratecauses a corresponding attenuation of waves impinging on the receivetransducer at a characteristic time period. Thus, the controller needonly detect the temporal characteristics of an attenuation to determinethe axial coordinate position. Measurements are taken along two axessequentially in order to determine a Cartesian coordinate position.

Other known systems, described in more detail below, employ a singlereflective array for separating as a plurality of wave paths, andsuperposing as a composite waveform, the signal from the transducer,through the active region, along a plurality of paths and then back tothe transducer, by providing an acoustically reflective edge spacedparallel to the reflective array, causing the dispersed wave to traversethe active region twice, as shown in FIG. 5. See, U.S. Pat. No.5,177,327, FIG. 10 and accompanying text, incorporated herein byreference.

FIG. 11 of U.S. Pat. No. 4,700,176 teaches the use of a singletransducer for both transmitting the wave and receiving the sensingwave, with a single reflective array employed to disperse and recombinethe wave. Such systems therefore employ a reflective structure oppositethe reflective array. As a result, an acoustic wave passes through theactive region twice, with consequent increased wave absorption by thetouch but also increased overall signal attenuation due to thereflection and additional pass through the active region of thesubstrate. Thus, the acoustic wave may be reflected off an edge of thesubstrate or an array of 180° reflectors parallel to the axis of thetransmission reflective grating and reflected back through the substrateto the reflective array and retrace its path back to the transducer. Thetransducer, in this case, is time division multiplexed to act astransmitter and receiver, respectively, at appropriate time periods. Asecond transducer, reflective array and reflective edge are provided foran axis at right angles to allow determination of a coordinate of touchalong perpendicular axes.

A known system by Electro-Plasma (Milbury Ohio) employs a bisectedreflecting array in order to reduce an acoustic wavepath, as shown inFIG. 6A. Therefore, a maximum path length of an acoustic wave along thecomposite reflecting array from a transducer is about one half of thetotal width, with transducers each sending acoustic waves toward thebisection point. Thus, the orthogonal set of paths will be longer, witha maximum total path length of two times the height plus the width. Inthis system, transmitting transducers are excited individually andproduce identical types of waves, portions of which travel alongparallel paths, with a small overlap of acoustic wave coverage of thetouchscreen in order to avoid a dead zone in the touch region. Theacoustic waves follow the traditional paths corresponding to axesparallel to the Cartesian coordinate axes. A similar type system wouldbisect both sets of reflective arrays, as shown in FIG. 6B.

The "triple transit" system, shown in FIG. 8, provides for a singletransducer which produces a sensing wave for detecting touch on twoorthogonal axes, which both produces and receives the wave from bothaxes. In this case, the area in which touch is to be sensed is generallyoblong, such that the longest characteristic delay along one path isshorter than the shortest characteristic delay along the second path,thereby allowing differentiation between the two axes based on time ofreception. See, U.S. Pat. Nos. 5,072,427, 5,162,618, and 5,177,327,incorporated herein by reference. The maximum path length of the tripletransit design is four times the width plus two times the height. Due tothe significant difference in path lengths, the X and Y signals arenon-overlapping, as shown in FIG. 9C.

d. Controller Algorithms

The wave pattern of one type of known acoustic touch sensors isdispersed along the axis of the transmitting reflective array, traversesthe substrate and is recombined, e.g., by another reflective grating,into an axially propagating wave, dispersed in time according to thepath taken across the substrate, and is directed to a receivingtransducer in a direction antiparallel to the transmitted wave, whichreceives the wave and converts it into an electrical signal forprocessing based on signal amplitude received as a function of time.Thus, according to this system, only two transducers per axis arerequired. Because of the antiparaliel path, the time delay of aperturbation of the electrical signal corresponds to a distance traveledby the wave, which in turn is related to the axial distance from thetransducer along the reflecting arrays traveled by the wave beforeentering the active area of the substrate, i.e., approximately two timesthe distance along the axis of the array plus the spacing between thearrays. A typical set of return waveforms is shown in FIG. 9.

The location of a touch is determined by detecting an attenuation of thereceived signal amplitude either in absolute terms or as compared to astandard or reference received waveform. Thus, for each axis, a distancemay be determined, and with two orthogonal axes, a unique coordinate forthe attenuation determined. Acoustic touch position sensors of this typeare shown in U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176,4,746,914 and 4,791,416, incorporated herein by reference.

U.S. Pat. Nos. Re. 33,151, and 4,700,176 also disclose a touch sensorsystem having a set of diverging acoustic paths which are incident on areflective array having elements located along an arc and spaced to meetcoherency criteria. See, Re. 33,151, and 4,700,176, FIG. 16 andaccompanying text, incorporated herein by reference. This touch sensorproduces a unidimensional output which corresponds to an angularposition of a touch.

According to known systems, a number of algorithms are employed todetermine the coordinate position of a touch. The simplest algorithm isa threshold detection, in which an amplitude of a received signal iscompared to a set value. Any dip below that value is consideredindicative of a touch. More sophisticated is an adaptive threshold, inwhich the threshold varies based on actual sets of received data, thusallowing increased sensitivity and rejection of artifacts of limitedamplitude.

A control circuit may operate in a number of modes, e.g., number oftransducers and configuration. In known systems having a rectangularsubstrate without redundancy, the number of transducers varies: 1(triple transit); 2 (ExZec/Carroll Touch); 4 (Adler); and 6(ElectroPlasma). There is a natural 8 transducer arrangement, notpresent in prior art designs, which is an extension of 6 transducerscheme in which 4 transducers are used for both X and Y axismeasurements; see FIG. 6B.

Known systems also include an adaptive baseline, in which an amplitudeof the normal received signal over time is stored, and the receivedsignal is compared to a baseline having a characteristic timeframe. Inthis system, an artifact in one position does not necessarily reducesensitivity at another.

Brenner et al., U.S. Pat. No. 4,644,100 relates to a touch sensitivesystem employing surface acoustic waves, responsive to both the locationand magnitude of a perturbation of the surface acoustic waves. Thesystem according to U.S. Pat. No. 4,644,100 is similar in execution tothe system according to U.S. Re. 33,151, while determining an amplitudeof a received wave and comparing it to a stored reference profile.

In order to reduce the number of transducers, the known "triple transit"system reflects the acoustic signal so that a wave emitted by a singletransducer is dispersed as parallel waves along a first axis, thenreflected at a right angle and dispersed as parallel waves along asecond axis. These waves are then reflected back to the arrays and thenback to the transducer, so that all the waves traveling along the firstaxis are received by the transducer prior to any waves traveling alongthe second axis, generally requiring an oblong substrate. The controllertherefore sets two non-overlapping time windows for the received signal,a first window for the first axis and a second window for the secondaxis. Therefore, each time window is analyzed conventionally, and thepair of Cartesian coordinates is resolved.

A system for sensing a force of a stylus against an acoustictouch-sensitive substrate is disclosed in U.S. Pat. No. 5,451,723,incorporated herein by reference. This system converts the point-contactof the rigid stylus portion into an area contact of an acousticallyabsorptive elastomer, placed between the stylus and the substrate.

e. Wave Modes

"Surface acoustic waves" ("SAW"), as used herein refers to acousticwaves for which a touch on the surface leads to a measurable attenuationof acoustic energy. Several examples of surface acoustic waves areknown.

The vast majority of present commercial products are based on Rayleighwaves. Rayleigh waves maintain a useful power density at the touchsurface due to the fact that they are bound to the touch surface.Mathematically, Rayleigh waves exist only in semi-infinite media. Inpractice it is sufficient for the substrate to be 3 or 4 wavelengths inthickness. In this case one has quasi-Rayleigh waves that are practicalequivalents to Rayleigh waves. In this context, it is understood thatRayleigh waves exist only in theory and therefore a reference theretoindicates a quasi-Rayleigh wave.

Like Rayleigh waves, Love waves are "surface-bound waves". Particlemotion is vertical and longitudinal for Rayleigh waves. Both shear andpressure/tension stresses are associated with Rayleigh waves. Incontrast, particle motion is horizontal, i.e. parallel to touch surface,for Love waves. Only shear stress is associated with a Love wave. Othersurface-bound waves are known.

Another class of surface acoustic waves of possible interest inconnection with acoustic touchscreens are plate waves. Unlikesurface-bound waves, plate waves require the confining effects of boththe top and bottom surfaces of the substrate to maintain a useful powerdensity at the touch surface. Examples of plate waves include symmetricand anti-symmetric Lamb waves, zeroth order horizontally polarized shear(ZOHPS) waves, and higher order horizontally polarized shear (HOHPS)waves.

The choice of acoustic mode affects touch sensitivity, the relativetouch sensitivity between water drops and finger touches, as well as anumber of sensor design details. However, the basic principles ofacoustic touchscreen operation are largely independent of the choice ofacoustic mode.

f. Optimization for Environmental Conditions

The exposed surface of a touchscreen is ordinarily glass. While certainsystems may include such additions, electrically conductive coatings orcover sheets are not necessary. Therefore, acoustic touchscreens areparticularly attractive for applications which depend on public accessto a durable touch interface.

Semi-outdoor applications, e.g., ATMs, ticket booths, etc., are ofparticular interest. Typically in such applications, the touchscreen isprotected from direct environmental precipitation contact by a booth oroverhang. However, indirect water contact, due to user transfer orcondensation is possible. Thus, users coming out of the rain or snowwith wet clothes, gloves or umbrellas are likely to leave occasionaldrops of water on the touchscreen surface. Water droplets have a highabsorption of Rayleigh waves in known systems; thus, a drop of water inthe active region will shadow the acoustic paths intersecting that drop,preventing normal detection of a touch along those axes.

One approach to limit water contact with the touchscreen surface is toemploy a cover sheet. See U.S. Pat. No. 5,451,723. However, a coversheet generally reduces the optical quality of the displayed image seenthrough the resulting sensor and leads to a less durable exposedsurface. Another approach to reducing the effects of water droplets isto employ a wave mode which is less affected by the droplets, such as alow frequency Rayleigh wave, see U.S. Pat. No. 5,334,805, a Lamb wave,see U.S. Pat. Nos. 5,072,427 and 5,162,618, or a zero order horizontallypolarized shear wave, see U.S. Pat. No. 5,260,521. These waves, however,also have reduced sensitivity, resulting in either reduced touchsensitivity of the touch system, increased susceptibility toelectromagnetic interference, or more expensive controller circuitry.

In the case of Rayleigh waves, a lower frequency operation requires athicker substrate, e.g., 3 to 4 wavelengths of the wave, and widerreflective arrays and transducers. The increased bulk of a sensordesigned for low-frequency Raylecigh waves is typically a seriousmechanical design problem. In the case of Lamb waves, a thin substrateis required, e.g., about 1 mm at about 5 MHz. These thin substrates arefragile, and Lamb waves have energy on both top and bottom surfaces,making optical bonding problematic due to signal damping. In the case ofa ZOHPS wave, in contrast to a Rayleigh wave, the relative sensitivityis greater to a finger than to water droplets. Further, ZOHPS wavessupport limited options for optical bonding, such as RTVs (siliconerubbers) which do not support shear radiation damping.

Shear sensors have two disadvantages in cold climates. In particularlycold climates, it is important for touchscreens to sense touches offingers of gloved hands. Shear waves have reduced sensitivity comparedto Rayleigh waves thus making detection of gloved fingers moredifficult. Secondly, in such climates, drops of water may freeze to formsolid ice. While liquid water does not strongly couple to horizontallypolarized shear waves, ice does. Thus drops of water which freeze on thetouchscreen surface will cause shadowing or blinding.

There remains a need for a touch position sensor which operates reliablyin the increasingly rugged environments to which such devices aredeployed. There thus exists a need to supplement existing technologiesin order to extend the applicability of acoustic touch sensor systems.

g. Size Constraints.

Acoustic sensors of the Adler type have been considered for use inelectronic white boards; see FIG. 10 and associated text in E.P.Application 94119257.7, Seiko Epson. At present, no commercialelectronic whiteboard products are available based on acoustic sensortechnology. In part, this is because of size limitations for knownacoustic technology.

SUMMARY OF THE INVENTION

The present invention derives from an understanding that acousticposition measurement technology suffers from various limitations, whichmay be addressed by implementing a system with various forms of partialredundancy in the sensing waves. Thus, for each coordinate axis of theoutput, a plurality of sets of waves are provided bearing informationabout the position of a single touch along that axis. Therefore, anylimitation in the ability of one set of waves to determine a touchposition may be supplemented by information derived from at least oneother set of waves. Because the redundancy may be partial, otherinformation may be derived from the available sets of waves as well,including a characteristic of a touch and information relating to aplurality of touches.

According to one set of schemes for producing partially redundant setsof waves, a plurality of sets of waves are provided, each propagating ata different angle with respect to the axis along which a touch positionis to be sensed. Each of the waves should be able to sense positionalong a significant portion of the axis. Thus, a traditional type touchsystem provides two sets of waves which are each parallel to an edge ofa rectangular substrate and produce waves which propagate perpendicularto the edges. Thus, each set of waves is dedicated sensing a positionalong a particular axis. Likewise, a known bisected reflective arrayscheme overlaps waves over an insignificant portion of the touchsensitive surface, and the waves generated are of the same frequency,mode, axis of propagation and therefore are essentially fully redundantand likely bear essentially the same information.

The present invention also extends these same principles to encompass anumber of other embodiments, including acoustic touch systems in whichthe acoustic waves travel along paths which are neither parallel norperpendicular to an edge of a substrate or travel along a path which isneither parallel nor perpendicular to a reflective array. Thus, thepresent invention relaxes constraints imposed in prior touch positionsensors through an understanding that the geometry of the touch sensorsubstrate. reflective arrays or acoustic paths need not limit thecoordinate system represented in an output. Thus, the present inventionmay provide control systems which are capable of performing coordinatesystem transforms and higher levels of analysis of the informationcontained in the acoustic signals than prior systems.

In forming this understanding that a control need not be limited to aconversion of a characteristic timing of a perturbation of an acousticwave into a coordinate position along a single axis, the possibility ofnon-Euclidean geometric shapes is developed. Thus, while the prior artteaches that acoustic touch sensing may be applied to spherical portionsof CRT faceplates, the goal of the prior art was to provide a system inwhich analysis of the received acoustic signals were as if the substratewere planar. Therefore, those prior art systems were developed tocompensate for the spherical aberrations in the design and placement ofthe reflective arrays. Likewise, a known prior art system employs adiverging set of waves incident on a reflective array to sense aunidirectional angular measurement. In this case, a control treats theunidimensional angular measurement as a single coordinate axis withouttransformation.

The present invention provides touch system flexibility allowinganalysis of waves which propagate along non-orthogonal axes in the touchsensitive region of the touchscreen. Further, the present inventionprovides a touchscreen system which tolerates and analyzes waves whichare overlapping in time, i.e., simultaneously impinging on one or morereceiving transducers. Together, these related aspects of the inventionprovide greatly enhanced flexibility in the design of the touchscreen,with improved performance under adverse conditions.

The present invention also includes touch sensors for purposes otherthan graphic user interfaces. For example, applications in the field ofrobotics exist, in which it is desirable to endow robots with a sense oftouch. While a number of sensor technologies exist, acoustic sensingprovides an opportunity for a large area, high resolution, low cost perunit area sensor on a machine, for example, to detect contact orpressure with an adjacent object and to determine the location of thetouch. Such machines often have nonplanar surfaces, and as such it isadvantageous to provide a touch position and/or pressure sensor whichconforms to the shell of the machine. According to the presentinvention, various surfaces having irregular geometries may be formedinto sensor surfaces.

The present invention also provides a touch system allowing analysis ofa wave perturbation of two different types of waves, the waves differingin mode, frequency, waveform, velocity, and/or wavelength. This systemadvantageously allows redundant position measurement and/or differentialwave perturbation sensing.

One aspect of the invention can also be described as follows. Acousticenergy is emitted into a substrate supporting propagation of acousticwaves. This energy travels through a portion of the substrate to areceiving system, which may include redundant use of the acoustic energyemitting device. The energy is received as at least two distinct waves.These waves have differing paths or characteristic timing. These wavesare non-orthogonal in either the time or space planes, meaning that theyimpinge simultaneously on one or more receiving transducers, or followpaths which are substantially non-orthogonal (having a relationdifferent than 90°).

Therefore, one embodiment of the present invention, as depicted in FIG.7, is somewhat similar to the "triple transit" system, but allowsacoustic signals following two different paths 1, 2 to be receivedsimultaneously. This system provides a first path 2 with a singlereflective array 5, which reflects acoustic waves off an opposite side 3of the substrate 4, back through the touch sensitive region of thesubstrate, back into the reflective array 5, and to the originatingtransducer 6, with a maximum path length of about two times the sum ofthe height plus the width. The orthogonal axis receives a portion of thesame acoustic wave from the transducer 6, which reflects off a diagonalcorner reflector 7, along a perpendicular axis has a second reflectivearray 8. The wave is reflected as a set of waves 9 through the touchsensitive region of the substrate 4, and is incident on a thirdreflective array 10, which reflects the acoustic wave toward a secondtransducer 11 on an adjacent side of the substrate 4, near the firsttransducer 6. The maximum path length of this path is two times the sumof the height plus width. In this case, two transducers 6, 11 receivesignals simultaneously for at least some delay times.

Another embodiment of the invention provides a sensor which employs aplurality of waves having differing frequencies, wavelengths, phasevelocities, or amplitude. Such waves may also be non-orthogonal in thetime or space planes, but need not be so. In other words, thesedistinguishable waves may travel sequentially and/or over orthogonalpaths.

Where portions of acoustic waves are received simultaneously by a singletransducer, it is generally preferred that a receiving circuit besensitive to a phase of a received signal in order to help resolveinterference effects. Likewise, where waves of differing frequencies areemployed, it is preferred that the receiver selectively receive thosewaves according to their frequency. Where waves of differing wavepropagation mode are employed, transducer having selectivity fordiffering waves modes may be provided. Therefore, embodiments of thepresent invention may also include a receiver sensitive to at least somewave characteristics.

A further embodiment of the invention provides a positive responsesensor, e.g., one where an increase in received signal is representativeof a typical perturbation. Typically, a perturbation in a positiveresponse system will cause a change of some type in the wave, making itdistinguishable from an unperturbed wave. Again, such a wave may benon-orthogonal in the time or space planes, but need not be so. Forexample, the unperturbed signal may be completely attenuated throughfiltering, and therefore not received by the receiver. In this case,only a single, positive response signal according to the presentinvention is received.

Thus, the present invention is not limited in the conventional manner tosequential receipt of independent coherent signals representative ofwaves propagating along Cartesian coordinate axes, and analysis thereofto determine an attenuation of a transmitted wave by a touch bydetecting the energy of the wave with respect to time. In particular,according to the present invention, a plurality of waves may be receivedsimultaneously, the received signal may be an incoherent superpositionof components from different wave sets, the waves need not propagateparallel to a rectangular coordinate axis of a planar substrate, anddetection is not necessarily based solely on a determination of a timeof an attenuation in power of a received signal. An improved receiver istherefore employed which includes enhanced logical analysis of thereceived waveform. Advantageously, the waveform sensitive analysis andenhanced logical analysis may be employed together.

The receipt of at least two distinct waves which overlap temporally mayindicate two waves which each have substantial energy, each beingspecifically intended for receipt, and potentially bearing informationrelating to a touch position along a coordinate axis. Alternately, oneof the two distinct waves may be due to unintentionally scattered waves,artifacts and interference that are not intended for use in touchdetection. In either case, a touch-information carrying signal may beutilized even if superposed with other signal components.

The present invention allows receipt and analysis of partially redundantwaves. Therefore, the effects of contamination and various artifacts maybe reduced. Further, where differing wave modes or frequencies are used,a differential sensing approach may be followed to determine bothposition and a mode sensitive characteristic of a touch.

The present invention includes a system in which the position of a touchis determined by the controller independent of the physical axes of thesubstrate, thus providing for coordinate processing and transformationbefore output. This allows increased flexibility in the layout of thetransducer systems. In this document, "transducer system" is defined tobe the system that couples electronic signals to acoustic waves in thedesired touch region including the transducer itself, e.g. a wedge oredge transducer, and associated reflective arrays if employed.

The present invention also allows receipt and analysis of signals whichare excited by a common transducer representative of differing sets ofwave paths with overlapping characteristic time-periods.

A still further aspect of the invention provides an acoustic wave touchsensor in which a touch is detected by a perturbation of a receivedsignal where the perturbation may be a decrease in amplitude, anincrease in amplitude, a change in phase of the received signal, or acombination of amplitude change and phase change.

One set of embodiments according to the present invention includessystems employing multiple waves sharing a common path portion. Theknown triple transit transducer also shares common path portions, butdoes not have simultaneously received waves or a transformation ofcoordinate system. In other words, the known triple transit systemrequires a time separation between received waves representingorthogonal axes, thus limiting the topology of the sensor.

According to one aspect of the present invention, a plurality of wavestraveling along non-orthogonal axes in the active region of thetouchsensor may have common path portions, being at least partiallysuperposed. In particular, according to certain embodiments of theinvention, these waves will share a common transducer, and a common axisof propagation from the transducer. The waves may differ, e.g., in path,mode, frequency, phase, propagation velocity, or wavelength. Therefore,some embodiments according to the present invention provide a reflectivearray which separates the waves to propagate along differing paths.Another set of embodiments provides a plurality of sets of distinguishedreflective arrays, which reflect portions of the waves at differingangles or as waves of differing propagation modes, or both.

Sensor systems according to the present invention allow superposition ofwaves producing sets of touch-sensitive waves which are dispersed acrossthe touch area of the substrate having characteristic time delays orother characteristics, and a system for receiving the dispersed wavesand determining a characteristic of a touch or wave perturbation. Theaxes of propagation of one set of waves need not be orthogonal to thoseof another set. According to the present invention, these sets ofnon-orthogonal waves may be employed with orthogonal waves. By providingmore than one set of these plurality of waves, a position of a touch maybe determined using redundant information, e.g., having more informationthan is necessary to mathematically determine a position, allowingenhanced performance in the presence of noise, interference andshadowing.

As stated above, the acoustic waves may differ in other properties,including mode, propagation velocity, wavelength, which in generalprovides two advantages. First, waves having differing properties mayhave differing sensitivity to environmetal conditions and artifacts.Thus, the differential effect on the sensing waves may be used todetermine properties of an object in contact with the surface. Further,the differences in the waves may be used to selectively filter thewaves, thus providing opportunity to selectively reduce noise orseparate potentially interfering waveforms. Waves having differingwavelength in the substrate may be selectively redirected withreflective arrays having physical characteristics corresponding to thatwavelength and its axis of propagation.

In another aspect of the invention, the coordinate system of a sensingwave is non-orthogonal with an output coordinate system. Therefore, aplurality of waves must be analyzed and their position informationtransformed in order to output a coordinate value. The plurality ofwaves may also be analyzed for redundancy to verify a touch coordinate,and potentially to resolve ambiguities, perhaps due to multiple touches,in the two dimensional position measurement.

In an embodiment of the invention, at least three distinct acoustic wavesets are excited, of which analysis of at least two are required inorder to detect a two dimensional position of a touch. Therefore, undervarious circumstances, one or more waves may be ignored or unavailable,yet operation continues. Where at least three are available, the threewaves may be analyzed for touch position consistency, artifact orinterference, and to determine an optimum output indication of theposition of the touch. The analysis of the at least three waves may alsoinclude an output of a plurality of simultaneous touch positions.

According to another embodiment, differing wave modes are induced in thesubstrate so that regions of low sensitivity employing one propagationmode correspond to regions which have adequate sensitivity employing adifferent propagation mode. For example, in regions where Rayleigh wavesare heavily shadowed due to contamination, a less sensitive backup wavemode, e.g. a horizontally polarized shear mode, may be analyzed for thissame region to determine touch data.

The dual mode operation allows operation with at least two waves, withspatial domain, frequency domain, wave propagation mode or time domainmultiplexing. Therefore, signals may be received along differing paths,having differing frequencies, differing wave propagation modes, ordiffering locations of reception.

In order to provide waves having differing characteristics fromsubstantially common sensor hardware, the signal from the transducersystem may include a number of components. In order to provide frequencymode discrimination, the receiving system must distinguish betweenvarious received frequencies. With respect to a plurality of wave modes,either the differing wave modes must be converted to a single mode whichexcites the transducer, or the transducer must be sensitive to thevarious modes. With a time domain multiplexing system, readingsaccording to various wave modes are taken sequentially. In order todetect spatially separated waves, a separate transducer may be providedor the waves may be redirected to a common receiving transducer. Wheredifferent types of waves are superposed, a perturbation will typicallyhave a different characteristic time delay for the different waves,which is used to distinguish the particular wave.

Various embodiments of the invention analyze a potential ambiguity inthe received waveform. That is, two waves, following different paths,arrive at the same receiving transducer within an indistinguishable timewindow, and thus a given wave perturbation is potentially attributableto either wave. Therefore, without further information, the controllermight not determine, based on the signal of the received wave, which ofthe two possible paths the touch intersects. According to a subclass ofthese embodiments, however, a pair of such ambiguous signalperturbations occur. Thus, by analyzing the pair of ambiguous signalperturbations, with reference to a physical model of the sensor andadditional information from signals from other wave sets, the positionof the perturbation may be determined or predicted, and the ambiguityresolved. Further, as referred to herein, the position may be sensedunambiguously by a pair of acoustic waves emitted along a single set ofsuperposed arrays.

According to another aspect of the invention, additional information maybe obtained from an additional set of superposed arrays, e.g., alonganother axis. This information may be further employed in determinationof the coordinate position. More generally, the present inventionencompasses the superposition of reflective arrays, e.g., to scatter aplurality of waves coherently, and a physically superposed arraystructure.

Where the waves travel along different paths, often the waves will bedirected towards different edges of the substrate. Therefore, forexample, two waves may be sensed with two different receivingtransducers simultaneously. Advantageously, therefore, a traditionaltouchscreen system and a touchscreen system with inclined propagationpaths are superposed. Embodiments according to the present invention maythus provide multiple channels for receiving acoustic wave information.

Reliability of operation is enhanced according to the present inventioneven where different types of touching objects are to be sensed, such asfingers, gloved fingers, styli, etc. Likewise, potentially interferingfactors may be identified and/or filtered or ignored.

By allowing multiple wave modes and/or paths, the advantages ofpartially redundant measurements and differential wave perturbationcharacteristic sensing are realized.

Algorithms may use redundant coordinate information to verify andperhaps resolve ambiguities in two-dimensional position measurements.

Alternately, algorithms may support systems with redundant coordinatemeasurements in which only two of three or more sets of waves are neededto reconstruct two-dimensional coordinates of a touch.

Systems according to the present invention also encompass multiple usertouch applications. In such cases, the redundancy of the multiple wavepaths may be used to resolve multiple degrees of freedom. For example,such multiple user touch systems might include a classroom multi-mediadevice that is simultaneously hands-on for the teacher and severalstudents, interactive museum displays, a two-person video game with atouch interface, or a large table-top display of engineering drawingsthat can be simultaneously reviewed and edited by a small group ofengineers. Complex control system human interfaces are also possible.

A multiple-user touch/display system will typically require a largerdisplay device than a system intended for one user at a time, such as ispossible with projection systems and large flat panel displayscommercially available or under development. Therefore, various methodsaccording to the present invention allow sensing of multiple touches,reduction of acoustic path lengths and of likely sources ofinterference. Embodiments according to the present invention also employaspects which allow longer acoustic path lengths.

Simultaneous touches are problematic for existing touchscreen products.Analog resistive, capacitive, and force-sensing touch technologiesinherently confound a multiple touch with a false touch at anintermediate position. High-resolution resistive and capacitive touchschemes that cover larger areas with discrete touch zones become awkwardand expensive due to the large number of electronic channels required.If means are provided to resolve discrete ambiguities, acoustic touchtechnologies have the inherent capability to recognize simultaneousmultiple touches. In addition to true multiple user applications, asimultaneous touch capability would also enable touch applications inwhich a single user simultaneously touches with both hands or more thanone finger in one hand. For example, a virtual piano keyboard on atouch/display device that supports playing of chords.

It is therefore an object of the invention to provide a system in whichwaves of differing characteristics are used to sense a touch in asubstrate, wherein the waves may have differing, non-orthogonal axes ofpropagation, differing wave propagation mode, differing frequency,wavelength or phase velocity.

It is another object according to the present invention to provide atouch sensor comprising an acoustic wave transmissive medium having asurface and a touch sensitive portion of said surface; a transducersystem for emitting acoustic energy into said medium; and a receiversystem for receiving the acoustic energy from the substrate, fordetermining a perturbation of said acoustic energy due to a touch onsaid surface, said touch sensor comprising a reflective array having aplurality of spaced elements for scattering portions of an incidentacoustic wave as waves having a different propagation vector than saidincident wave and passing other portions unscattered, said array beingprovided an array selected from the group consisting of:

(a) an array associated with said medium situated along a path, saidpath not being a linear segment parallel to a coordinate axis of asubstrate in a Cartesian space, a segment parallel to an axial axis orperpendicular to a radial axis of a substrate in a Cylindrical space,nor parallel and adjacent to a side of a rectangular region of a smallsolid angle section of a sphere;

(b) an array situated along a path substantially not corresponding to adesired coordinate axis of a touch position output signal;

(c) an array situated along a path substantially non-parallel to an edgeof said medium;

(d) has a spacing of elements in said array which differs, over at leastone portion thereof, from an integral multiple of a wavelength of anincident acoustic wave;

(e) has elements in said array which are non-parallel;

(f) has an angle of acceptance of acoustic waves which varies overregions of said array;

(g) coherently scatters at least two distinguishable acoustic waveswhich are received by said receiving system; and

(h) combinations and subcombinations of the above,

except that said array in (d), (e) or (f) is not provided parallel andadjacent to a side of a rectangular region of a small solid anglesection of a sphere.

It is also an object according to the present invention to provide acontroller which is capable of logically analyzing two sets of wavesderived from a common transmit transducer burst which are receivedsimultaneously, i.e., in which the wave being received may not bedistinguished solely by reference to a time window. Thus, the systemneed not maintain a time separation between a plurality of waves forproper operation.

It is a still further object of the invention to provide a receiver inwhich a received signal is analyzed for waveform information, due e.g.,to multipath signal paths. Further, the receiver according to thepresent invention may analyze the received signal for a touch indicatedby a perturbation of complex amplitude rather than merely an attenuationin received power.

A further object according to the present invention is to allow outputof a coordinate position in an output coordinate system, typicallyCartesian, of a perturbation of acoustic waves, each of which measures acoordinate substantially different from the axes of the outputcoordinate system.

Thus, touch position sensors according to the present invention mayprovide some or all of the following advantages:

(a) Tolerance to shadowing effects of contaminants by obtainingredundant information and/or employing robust waveforms.

(b) A higher signal to noise ratio due to availability of redundantcoordinate information.

(c) A multiple wave mode sensor allowing the composite advantages ofeach selected type of wave mode, e.g., high sensitivity to touch forRayleigh wave modes, relative immunity to contaminants for horizontallypolarized shear modes.

(d) Ability to detect a mode sensitive perturbing characteristic of atouch based on differential wave perturbation and/or appearance of acharacteristic new signal.

(e) Versatility in the selection of substrate, e.g., use of largersizes, non-rectangular shapes, large solid angle sections of spheres andother non-planar topologies.

(f) Ability to reliably reconstruct multiple touches and hence supportapplications in which more than one finger, hand, or user maysimultaneously input touch information.

These and other objects will become apparent from a review of thedrawings and Detailed Description of the Preferred Embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will be explained byreference to the drawings, in which:

FIG. 1 is a prior art touchscreen system having a plurality each oftransmitting transducers and receiving transducers;

FIG. 2 is a prior art touchscreen system having transmitting transducerswhich emit a wide acoustic wave;

FIG. 3 is a prior art touchscreen system having a diverging transmittingtransducer with a single reflective array along a curved path;

FIG. 4 is a prior art touchscreen system having four transducers andfour reflective arrays;

FIG. 5 is a prior art touchscreen system having two transducers and tworeflective arrays;

FIG. 6A is a prior art touchscreen system having six transducers, twonormal arrays and two segmented reflective arrays

FIG. 6B is a touchscreen system having eight transducers and foursegmented reflective arrays;

FIG. 7 is a touchscreen system having two transducers and threereflective arrays;

FIG. 8 is a prior art triple transit touchscreen system having onetransducer and two reflective arrays;

FIGS. 9A and 9B show typical waveforms received from a transducer systemaccording to FIG. 4 subject to one and two simultaneous touches,respectively;

FIG. 9C shows a typical waveform received from a transducer systemaccording to FIG. 8;

FIG. 9D(1) shows the superposition of two simultaneously receivedsignals, as would be seen by summing the receiving transducers of theembodiment of FIG. 7;

FIGS. 9D(2), 9D(3) and 9D(4) show the sum of the superposed signals ofFIG. 9D(1) when the superposed signals are (2) in phase, (3) out ofphase with an RMS detector, and (4) out of phase with a phase preservingreceiver, respectively;

FIG. 10 shows a generic planar sensor subsystem, without intermediatereflections;

FIG. 11 shows the array reflector spacing and orientation of anembodiment according to FIG. 10;

FIGS. 12(a)-(f) show example coordinate subsystem geometries asembodiment of the touchscreen system according to FIG. 10, of which FIG.12(a) is prior art;

FIGS. 13(a) and (b) show a rectangular touchscreen according to thepresent invention having two orthogonal sets of wavepaths and anadditional diagonal set of wavepaths, respectively;

FIG. 14 shows a rectangular touchscreen according to the presentinvention having two orthogonal sets of wavepaths and two diagonal setsof wavepaths of which the second set of diagonal wavepaths is shown;

FIG. 15(a) shows a detail of a reflective array according to the presentinvention having two significant Fourier transform components supportingtwo distinct spacing vectors for two distinct coordinate subsystems;

FIGS. 15(b) and 15(c) show hexagonal and triangular sensor systemsaccording to the present invention respectively incorporating reflectivearrays according to FIG. 15(a);

FIG. 16(a) shows a large area rectangular sensor according to thepresent invention having segmented reflective arrays;

FIGS. 16(b) and 16(c) show linear segmented reflective arrays andshingled segmented reflective arrays, respectively, according to thepresent invention;

FIG. 17 shows a generic planar sensor subsystem, similar to thesubsystem according to FIG., 10, with intermediate reflections;

FIGS. 18(a)-18(d) shows example touchscreen systems incorporating asensor subsystem according to FIG. 17, of which 18(c) with commontransmit/receive transducer system and 18(d) are prior art;

FIG. 19(a) shows an isometric view of a cylindrical sensor system havinga triple superposed array at one end thereof, according to the presentinvention;

FIG. 19(b) shows a planarized representation of the surface of thesensor system according to FIG. 19(a);

FIG. 20 shows a generic non-planar sensor subsystem, according to thepresent invention;

FIGS. 21(a) and 21(b) show top and side views, respectively, of aspherical section touchsensor system, according to the presentinvention, for determining a touch position in a spherical coordinatesystem;

FIGS. 21(c) and 21(d) show flat-map projection and top views,respectively of another embodiment of a hemispherical touchsensor systemaccording to the present invention;

FIGS. 22(a) and (b) show isometric and plan views, respectively, of aLove wave mode basin sensor according to the present invention;

FIG. 22(c) shows a side view of a half-hemisphere touch sensor having asingle transducer and reflective array system according to the presentinvention;

FIGS. 23(a)-(d) show respectively signals received from transducers ofthe embodiment according to FIG. 13;

FIGS. 24(a) and (b) show respectively a generalized representation ofand a detailed specific example of flow diagrams for a redundancy checkalgorithm according to the present invention;

FIGS. 25(a) and (b) show respectively a generalized representation ofand a detailed specific example of flow diagrams for an anti-Shadowingalgorithm according to the present invention;

FIG. 26 shows a generalized flow diagram for a differential touchcharacteristic sensing algorithm according to the present invention (seeFIG. 28(c) for a specific example);

FIGS. 27(a) and 27(b) show a touch position sensor system having twonon-orthogonal sets of wavepaths for waves of differing modes, and atiming diagram showing the relationship of received signals,respectively, according to the present invention;

FIGS. 28(a) and 28(b) show a touch position sensor system having twonon-orthogonal sets of wavepaths for waves of a single mode and arectangular set of wavepaths for a third wave, and a timing diagramshowing the relationship of received signals for two simultaneoustouches, respectively, according to the present invention;

FIGS. 29(a)-(d) shows diagrams of the effect of a perturbation of one ofa set of superposed waves with arbitrary phase relationship;

FIG. 30 shows a typical AM receiver circuit of prior art suitable forprocessing signals received by the embodiment according to FIG. 4;

FIGS. 31(a)-(f) show, respectively, alternative circuit diagramsaccording to the present invention for implementing a phase sensitivereceiver, having (a) a carrier synthesis circuit based on thetransmitted wave burst; (b) a tracking phase locked loop carrierrecovery circuit; (c) a carrier circuit which employs the transmit burstclock; (d) a clock recovery circuit based on the received signal; (e) adigital signal processor embodiment which oversamples the signal andemploys software to analyze the digitized signal; and (f) an applicationspecific integrated circuit embodiment which performs phase detection,filtering and decimation in time prior to transferring a digital signalrepresentation to a microcomputer; and

FIGS. 32(a)(1), 32(a)(2), 32(b), and 32(c) show flow diagrams ofportions of a sensor system control sequence demonstrating many aspectsof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The below detailed description and examples are provided to illustrateaspects and examples of the present invention, and should not beconsidered to limit various other possible combinations orsubcombinations of the elements. Therefore, it should be understood thatthe below examples are preferred embodiments or portions of embodimentswhich form a part of the invention, which is to be construed in view ofthe entirety of the specification, including relevant aspects of therecited prior art and the claims.

Overview Acoustic Waves

The wave employed for sensing touch may be any acoustic wave which isdetectably perturbed by a touch on a surface of a substrate. Manyoptions exist for the choice of surface acoustic wave modes. Rayleighwaves have excellent touch sensitivity and are inherently confined to athin volume close to the touch surface even for a substrate of anarbitrarily large thickness. Horizontally polarized shear waves have theadvantage that they weakly couple to liquid and gel-like contaminantssuch as water and silicone-rubber seals. A non-homogenous substrate maysupport asymmetric horizontally polarized shear waves, including Lovewaves, which are horizontally polarized shear waves trapped near thetouch surface like Rayleigh waves. Lamb waves in a sufficiently thinsubstrate provide yet another option for the choice of surface acousticwave mode. Various engineering trade-offs are involved in the optimalchoice of acoustic mode for a given application.

In this context, Love waves may be supported by a top substrate portionhaving a lower phase velocity interfaced with a lower substrate portionhaving a higher phase velocity. Similar types of waves, generallyclassified as asymmetric horizontally polarized shear waves, may besupported by vertical phase velocity gradients of a more complex nature.Asymmetric horizontally polarized shear waves have particular advantagesif they have essentially no power density on the lower surface. WhereLove waves are employed herein, asymmetric horizontally polarized shearwaves may be analogously employed.

The present invention seeks to enhance operation of touchscreens byproviding various types of wave redundancy, wherein the redundant waveis subject to differing interferences, tradeoffs and artifacts. Thus,one redundancy strategy provides waves which propagate along differingangles. Thus, a shadowing effect which reduces sensitivity along onewave propagation axis may not also shadow a wave passing along adifferent axis. A second strategy employs wave having differing modes ofpropagation, thus allowing the advantages of each mode to be exploited,while allowing differential detection to determine an absorptivitycharacteristic of a touching object. When the differing wave modestravel along differing axes as well, further robustness is obtained.

Use of differing frequencies increases the options for implementing waveredundancy. In addition to differentiating redundant waves viadifferentiating axis or modes, different sensor subsystems may bedistinguished by narrow band filters and filtering techniques. Inaddition, by operating at differing frequencies and differing modes,physical structures of the touchscreen may be redundantly employed toreduce substrate system complexity. Furthermore, a given acoustic mode,e.g. Rayleigh, at different frequencies will differ in characteristics,e.g. touch sensitivity and the amount of shadowing from contaminants, sothat use of different frequencies can serve similar purposes as the useof differing acoustic modes.

It is noted that typical known acoustic touchscreens employ twodifferent angles, e.g., X and Y axes. However, these are not consideredredundant except for the existence of a touch, rather than its position.Therefore, aspects of the present invention provide a system which iscapable of providing redundant information defining a position along atleast one coordinate axis, or a position along a coordinate axis whichdoes not correspond to an edge of the substrate.

Substrate

An acoustic touchscreen sensor is often constructed from a sheet-likesubstrate, which is a material capable of supporting propagation ofacoustic waves with rciatively low attenuation. Often, touchscreens aredisposed in front of display devices, and thus they are transparent. Thepresent invention allows the acoustic substrate to be an integralcomponent of a display device, such as the face-plate of a cathode raytube. The acoustic substrate may also be a touch-sensitive mechanicalsurface of a device, in the absence of an associated display device. Incircumstances where transparency of the substrate is not required, ametal (e.g., aluminum or steel) or ceramic substrate may be employed.

The substrate or a component of a substrate, e.g., a portion of anacoustic filter, may also be formed of plastic. It is noted that mostplastics are typically more acoustically absorptive than inorganicmaterials such as glass, and therefore may be unsuitable for use asmonolithic substrates for acoustic touchsensors of substantial size.However, for smaller devices, plastics may be employed as the substrateor a component of a substrate. Acoustic absorption varies greatly fordifferent polymer materials. Amongst plastics, polystyrene, low densitypolyethylene, Nylon 6/6, polypropylene, Lustran and acrylic haverelatively less acoustic absorption. For an all plastic substrate, useof such a relatively low-loss plastic is preferred. If plastic is usedto form a portion of an acoustic filter, then use of a higher-lossplastic may be permitted or even desired.

The substrate may be monolithic, laminated or coated. Non-monolithicsubstrates may he employed to alter a wave energy distribution in thesubstrate, support selected wave mode propagation, or to filterundesired wave modes. For example, a sandwich of a slow-velocity layeron a fast-velocity layer on an acoustically absorptive layer may supportLove waves and simultaneously filter out parasitic plate waves. Thus,the substrate may comprise layers having differing acoustic propagationproperties and/or acoustic interfaces.

In some cases, it may be convenient for manufacturing purposes tofabricate reflective arrays on strips of material which are then bondedonto the rest of the substrate; see U.S. Pat. No. 4,746,914, column 9.Thus a laminated structure may be used for manufacturing convenience orpackaging configuration. Such laminated strips may also achieve benefitsin acoustic performance.

A sheet-like substrate, in a touchscreen embodiment for a computer videodisplay monitor, is commonly 2 to 3 mm thick transparent soda-limeglass. It is noted that various substrates may provide particularadvantages for acoustic touchscreen sensors. For example, borosilicateglass has been found to have up to approximately 30 dB increasedsignal-to-noise ratio over soda lime glass for a typical Rayleigh modetouchscreen system for a display monitor. Further, the reduced rate ofacoustic attenuation in borosilicate glass is of increasing benefit asthe lengths of the acoustic paths increase. Accordingly, a borosilicateglass substrate may therefore be advantageously used for large dimensionsensor systems, or those with long acoustic wave paths.

One advantageous embodiment of the present invention provides aborosilicate glass substrate on the back of which is laminated, orotherwise provided, a projection screen. Note that the use of Rayleigh,Love and any other mode that has little energy on the back surface canbe conveniently used with a lamination or other acoustically absorptivestructure on the back surface. Known projection Systems include cathoderay tubes, liquid crystal shutter devices, electro-optic projection oflaser beams, and other spatial light modulators, such as a so-calleddigital mirror device ("DMD") from Texas Instruments. Borosilicate glassmay thus be advantageous for the large areas of image projectiondevices. In addition, the geometric flexibility of the present inventionenables considerable increase in the maximum feasible sensor size.Furthermore, such large devices may be placed in environments subject toenvironmental contamination, and therefore benefit from a robust touchsensing system as provided by the present invention, e.g., by useredundant or differential sensing of touch coordinates.

A large substrate may also be employed in a large white boardapplication, in which the substrate is touch sensitive over a largearea. In a white board application, the substrate need not betransparent, and therefore may be formed of an opaque material such asaluminum. Advantageously, aluminum and some other metals may be coatedwith an enamel with a relatively slow acoustic phase propagationvelocity, thus supporting a Love wave with high touch sensitivity(relative to horizontal shear plate-wave modes) on the front surface.

A touch sensor substrate suitable for Love waves, a shear-type wavehaving wave energy at one surface and substantially less on the opposedsurface, is formed as a laminate of two or more substrate materialswhich differ in phase velocity, with the lower velocity on the top,sensitive surface. The laminate may include a number of layers, whichfollow a generally increasing pattern of phase velocity change.Alternately, the laminations may have varying phase propagationvelocities, provided as a filter arrangement to particularly select adesired Love wave mode. Filtering by the substrate may be direct, e.g.,undesired modes are damped or evanescent, or indirect, phase velocitydifferences between desired mode and undesired modes are increased, thusenhancing the mode filtering performance of reflective arrays.Therefore, alternating layers of higher and lower phase velocitymaterials, with an overall asymmetry may be provided to help, e.g.,reflective arrays, select a desired Love wave. By selecting the phasevelocity distribution, the laminated substrate may selectively varyproperties of different wave modes.

An example of a Love-wave substrate with a lower velocity surface layer,is a 2 mm glass sheet having a uniform coating of lead-based frit of 0.1mm thickness. Other frit or glazing options may be appropriate forapplications where acid leaching of lead from the touch surface is apotential health hazard. It is possible to construct Love wavesubstrates that strongly capture acoustic energy near the surface andhence improve touch sensitivity. For example, a 100 micron thick layerof lead-containing enamel having a shear velocity of less than about 2.6mm/μsec, on top of a 2-3 mm thick aluminum sheet with shear velocityabout 3.0 mm/μsec, captures the majority of Love-wave power within 200microns of the surface; such a substrate is very similar to a knownenamel-coated architectural aluminum panel.

Further, a glazed ceramic will also support Love wave propagation,provided that the glaze has a reduced shear wave velocity relative tothe ceramic. Thus, for example, a basin may be formed as a plumbingfixture, e.g., a sink, or toilet bowl.

The substrate may be formed as a flat plate with a rectangular shape ora non-rectangular shape such as a hexagonal. Alternatively the substratemay be curved along one or both axes as a cylindrical, spherical orellipsoidal surface or section surface, or may have otherconfigurations. In particular, large solid angle spherical, and completecylindrical substrates are contemplated. This invention providesflexibility in the layout of transducers and arrays to accommodate sucha variety of sensor shapes. For example, a polygonal touch sensor may beprovided with reflective arlays on each side and transducers at eachvertex.

Cylindrical substrates are particularly interesting applications of thepresent invention, because perimeter reflective arrays may be usedwithout need for arrays parallel to the axis of the cylinder, allowingdetermination of both axial position and angle. Thus, a cylindricaltouch sensor may be used for electronically controlling focus and zoomof a camera lens in a cylindrical housing without external moving parts.By employing selected wave modes having low fractional absorption bywater and a significant differential absorption between water and flesh,e.g., horizontally polarized shear waves, applications such as anunderwater camera control may be supported.

Reflective Arrays

Spacing-vector Formula

Reflective arrays coherently scatter or redirect acoustic energy in acontrolled fashion. Referring to FIG. 10, general principles ofreflective array design are introduced below. These very boardprinciples enable engineering of novel reflective array designs.

A comment on notation: in this document, variables which are vectorquantities are given in bold face, while scalar variables are not.

Reflector orientations and spacings can be determined from the wavevectors of the desired incident and reflected acoustic waves. These wavevectors are defined as follows. Let the wave vector k_(I) be defined asparallel to the incident beam direction and as having a magnitude k_(I)=2π/λ_(I). The wavelength of the incident beam, λ_(I) =v_(I) /f, isdetermined from the operating frequency, f, and the phase (not group)velocity, v_(I), of the incident wave. Likewise for the reflected wave,let us define k_(R) be defined as parallel to the reflected beamdirection with magnitude k_(R) =2π/λ_(R) where λ_(R) =v_(R) /f withv_(R) being the phase velocity of the reflected wave.

These wave vectors may be a function of position along the array. Forthis purpose we introduce a path parameter "s" which uniquely identifiesmembers of sets of acoustic wave paths, and hence also the positionalong arrays at which the acoustic path is scattered from the transmitarray or scattered onto the receive array. For example each member ofthe set of acoustic paths can be represented by a value of "s" betweenzero and one. In general, the incident and reflected wave vectors for anarray are functions of s: k_(I) (s) and k_(R) (s). For a sensor systemas described in U.S. Pat. No. 4,645,870, in which the value of s for atouch is identified by the frequency of the signal absorption, even theoperating frequency, f(s), and hence the wavelengths, λ_(I) (s) andλ_(R) (s), depend on the path parameter. For notational convenience, thepath parameter dependence is not always explicitly written and isimplicitly implied. In many cases, such as flat rectangular sensors ofthe prior art, the wave vectors are constants that do not depend on thepath parameter.

The spacing and orientation of the array reflectors, for each positionalong a reflective array, can be determined from the wave vectors k_(I)and k_(R). Consider, for example, the case where the reflective array iscomposed of line-segment reflector elements; see FIG. 11. Of particularinterest is the reflector spacing vector S which is perpendicular to thereflector lines and has a length equal to the center-to-center spacingbetween reflector elements in a direction parallel to S. If thereflector spacing vector S is known, then so are the orientations andspacing of the reflectors.

It is noted that the line-segment reflector elements represented in FIG.11 are a form of pulse compression filter, and other known types of suchpulse-compression filters may be employed according to the presentinvention, as appropriate.

The spacing vector, which in general is a function of the pathparameter, may be determined from the incident and reflected wavevectors as follows.

    S=2πn(k.sub.I -k.sub.R)/|k.sub.I -k.sub.R |.sup.2

This is the fundamental equation of coherent reflective array design.All reflector element orientations and spacings found in the prior artcan be derived as special cases of this general formula. Furthermore,this fundamental equation enables the design of arrays for largefamilies of new sensor embodiments outside the scope of prior art. Givenits importance, the derivation of this spacing-vector formula ispresented is some detail below.

For the derivation, we define a reflector orientation vector R which isparallel to the reflector line elements and whose length is the distancefrom point A to point C in FIG. 11. The reflector spacing vector S isperpendicular to the reflector elements; in vector dot-product notation,this orthogonality condition is expressed as R·S=0.

Note, for example, that k_(I) ·R=(2π/λ_(I))×R×cos(θ) where θ is theangle defined by the points A, C, and F. R×cos(θ) is the extra distancethe incident acoustic wave must travel to intercept the reflector atpoint C rather than point A. Hence (2π/λ_(I))×R×cos(θ) is the phaseadvance, in radians, of the incident acoustic mode in propagating frompoint F to point C.

Similarly k_(R) ·R is the phase advance for the reflected acoustic modepropagating from points A to G.

Coherent scattering from arbitrary pairs of points within a givenreflector element, e.g. points A and C, requires equality of the phasedelays k_(I) ·R and k_(R) ·R. This requirement can be expressed as(k_(I) -k_(R))·R=0, that is, the reflector elements are perpendicular tothe vector difference of the incident and reflected wave vectors.

If there is no mode conversion, and hence the wave vectors k_(I) andk_(R) have the same magnitudes, (k_(I) -k_(R))·R=0 reduces theangle-of-incidence-equals-angle-of-reflectance rule familiar from theoptics of mirrors. If there is mode conversion, then the angle ofincidence no longer equals the angle of reflectance, but rather isquantitatively analogous to Snell's law familiar from refraction inoptics (where phase velocity equals the speed of light divided by theindex of refraction).

Note that because vectors S and (k_(I) -k_(R)) are both perpendicular tothe reflector elements, both are orthogonal to R. Hence they must beparallel to each other (the algebraic sign of ±S is physicallyirrelevant, so there is no loss of generality if we neglect thepossibility that they are anti-parallel). This confirms that thespacing-vector formula correctly gives the spacing vector direction.

Now consider the requirement that scattering between arbitrary points ondifferent reflectors be coherent. In particular, consider scatteringfrom points B and C in FIG. 11. The phase delay for the incident wavepropagating from points D to B can be shown to equal k_(I) ·S. The phasedelay for the scattered mode to propagate from points B to E is -k_(R)·S. Hence the total phase delay for scattering off of point B relativeto scattering off of point C is (k_(I) -k_(R))·S. To have coherentscattering between reflector elements, this total phase advance must bean integer multiple of 2π, and hence the condition (k_(I) -k_(R))·S=2πn.

Together, the within-reflector-coherence condition, (k_(I) -k_(R))·R=0,and the between-reflector-coherence condition, (k_(I) -k_(R))·S=2πn,tells us that (k_(I) -k_(R)) is parallel to S and has a magnitude of2πn/S. Hence we have the following equality.

    (k.sub.I -k.sub.R)=2πnS/S.sup.2

Solving for S gives the fundamental spacing-vector formula given above.

A given spacing vector S will support scattering in the reversedirection. More formally, if we define the reversc-direction wavevectors k_(I) '=-k_(R) and k_(R) '=-k_(I) then a spacing vector Ssatisfies the spacing-vector formula for k_(I) and k_(R) if and only ifit satisfies the spacing-vector formula for k_(I) ' and k_(R) '.

A special case of a spacing vector is an "n=1 spacing vector" whichsatisfies the following relation that does not contain a factor "n".

    S=2π(k.sub.I -k.sub.R)/|k.sub.I -k.sub.R |.sup.2

From a mathematical perspective, this is the most fundamental form ofthe spacing vector formula. Mathematically, a reflective array with aspacing vector of S is typically a superposition of a components withspacing vectors of S/2, S/3, etc. The n>1 solutions result from coherentscattering off of these "higher harmonics" of the basic reflectorspacing.

Definition of Reflective Array

Reflective arrays in known systems and available touchscreen productsare composed of line-segment reflective elements composed of materialdeposited on the substrate surface, fabricated by removing material fromthe substrate surface, or combinations of both. Here we define areflective array more generally to be a region of a sensor subsystem inwhich the acoustic properties of the medium have been modulated in sucha fashion to produce a distribution of scattering centers which has asignificant two-dimensional Fourier-transform amplitude for the point intwo-dimensional wave-vector space given by 2πS/S² where S is a desiredn=1 spacing vector. This condition assures coherent scattering in thedesired direction into the desired acoustic mode.

In a preferred embodiment, reflective arrays are composed of linearreflector elements as illustrated in FIG. 11. The reflective arrays maybe formed by any means that alters the acoustic impedance of thesubstrate for the incident acoustic waves. In a preferred embodiment,reflector elements are raised lines formed by screening glass fritsthrough a stencil on the surface of the substrate and subsequentlyfiring the screened substrate to fuse the frit to the substrate. Inanother embodiment, the reflectors are depressions in the acousticsubstrate, perhaps back-filled with another material. Manufacturing costconsiderations often guide the choice amongst the many options here.

While the general definition of reflective arrays includes arrays ofline-segment reflectors in which the shortest distance betweenreflectors is given by the desired n=1 spacing vector S or a multiplethereof, this is not a requirement. For example, in analogy with crystaldiffraction of neutrons off of nuclei, a reflective array may becomposed of an array of reflective dots. X-ray crystal diffractionprovides an analogy where the distribution of scattering centers(probability densities for electron orbitals) is more complex;reflective arrays need not be divisible into well-defined or isolatedreflective elements. For example, a ductile substrate surface, e.g.aluminum or temporarily heated glass, may be modulated in a sinusoidalor more complex fashion by a stamping process. The coherent scatteringprinciples in the spacing-vector formula are independent of the detailsof reflective array fabrication techniques.

Many embodiments involve superposed reflective arrays. The generaldefinition of a superposed array is a region of a sensor subsystem inwhich the acoustic properties of the medium have been modulated in sucha fashion to produce a distribution of scattering centers which hassignificant two-dimensional Fourier-transform amplitudes for two or morepoints in 2-D wave-vector space given by 2πS_(i) /S_(i) ² where Si fori=1,2, . . . are two are more desired n=1 spacing vectors.

As stated above, other types of pulse compression filters may beemployed to redirect or scatter portions of the acoustic wave alongdesired paths or sets of paths. Generalizing, therefore, the pulsecompression filter has at least one two dimensional Fourier transformwith an admittance for a desired wave or set of waves. Examples of theseother types of pulse compression filters are employed, for example, inthe field of piezoelectric radio frequency surface acoustic wave (SAW)devices.

Shared Wave Paths

In order to simplify the construction of the system or to support morecompact sensor designs, a plurality of desired waves may share physicalelements and portions of wave paths. In order to create a system inwhich a plurality of sensor subsystems share portions of wave paths, anumber of configurations are possible.

First, a reflective array having a set of reflective elements with acharacteristic spacing and angle which is suitable for scattering twodifferent waves may be employed to generate two sets of wave paths,which are ultimately analyzed. Thus, the same physical elements of thereflective array may be used to reflect both waves. Generally, suitablewaves may differ in frequency and/or wave mode. Referring to FIG. 11,this is the case in which a single spacing vector, S, simultaneouslysatisfies the spacing vector formula for two or more combinations ofincident and reflected wave vectors; the value of n need not be the samefor the two or more combinations. While this approach provides anelegant economy of design, the spacing-vector formula places constraintsthat limit the geometrical flexibility of this approach.

Second, a plurality of arrays may be provided, adjacent to each other,as shown in FIGS. 18(a), and (c). In this case, each array may beprovided with a separate transducer or a transducer wide enough toexcite acoustic waves propagating through more than one array. Thespatial separation of the arrays provides opportunity for filtering ofthe wave between the arrays to remove undesired components. A wave froma lateral array must pass through a medial array in order to travel toor from the central, touch sensitive region. Scattering from undesiredarray may lead to some loss of signal amplitude and perhaps generationof parasitic acoustic paths; typically this will be a minor effectbecause only a small portion of the wave will be scattered by each arraythe wave passes through. If necessary, various parasite suppressiontechniques discussed below may be applied. For applications with tightspace constraints, this approach has the disadvantage that the regioncontaining the arrays near the edge of the substrate may need to berelatively large.

Third, arrays may be placed on opposite faces of the substrate. In thiscase, the waves intended for interaction with the reflective arrays onthe back of the substrate must have a significant power density on therear surface of the substrate, as is the case for plate waves. Forexample, A reflective array on the top surface may have a spacing vectordesigned for Rayleigh-to-Rayleigh scattering, and a rear reflectivearray may have a spacing vector designed to couple a HOHPS wave in thetouch region to a back-side Rayleigh wave. This approach contrasts withthe top-and-bottom array schemes proposed for Lamb wave sensors in orderto distinguish symmetric and antisymmetric Lamb waves, allowing one toassure that only one acoustic mode is emitted; see U.S. Pat. Nos.5,329,070, 5,243,148, 5,072,427, 5,162,618, and 5,177,327.

Fourth, a plurality of reflective elements arranged as arrays may besuperposed in the same physical space, or more generally the reflectivearray may be designed by whatever means to support more than one spacingvector, as shown in FIG. 15(a). This configuration is generallypreferable, because of its efficient use of spacing, efficient use oftransducers and corresponding wiring and electronics. If the differingacoustic modes propagate along the superposed arrays, e.g., Rayleigh andshear, distinct transducers may be required. Advantageously, where thearrays are physically superimposed and only a single mode is propagatedalong the array, a single transducer may be employed.

Reflective Boundaries as Example of Reflective Array

The acoustic path may encounter a reflective boundary betweenscatterings off of the transmit and receive arrays. This is thegeneralized concept shown in FIG. 17. The reflective boundary mayutilize coherent scattering from a superposition of scattering centers,and if so can be designed using similar principles as for reflectivearrays that follow segments of acoustic paths. For example, the spacingvector formula can still be applied. Note, however, that for reflectiveboundaries, it may be advantageous to use reflective elements thatscatter more strongly.

In many cases, a reflective boundary enables common or superposedtransmit and receive arrays. For example, consider a superposedreflective array on a flat substrate including elements disposed atangles of 45°±θ. Thus, waves propagating along the array will bescattered at angles of 90°±2θ. Where a reflective structure is disposedopposite and parallel to the superposed reflective array across thesubstrate, and the reflective boundary's spacing vector is perpendicularto the boundary, waves will travel in sets triangular paths.

Under certain circumstances it may be desired to alter a wave mode whilereflecting from a reflecting structure, rather than at the reflectivearray. In this case, the reflective structure may be provided as aseries of appropriately spaced parallel elements which togetherefficiently scatter the wave energy as a selected mode; thecoherent-scattering condition of FIG. 11 applies to reflectiveboundaries as well as reflective arrays. For example, a variant of thetriangular acoustic paths above is one in which the reflective boundaryand one of the sets of reflectors of the reflective array are designedas mode converters.

Parasitic Acoustic Paths

Engineering care may sometimes be required in array design to minimizecreation of undesired parasitic acoustic paths which can result insignal artifacts. There are many means available to suppress suchparasitic signals.

The reflective arrays serve as narrow band filters for both wavelengthand angle of propagation. Thus, a reflective array has a highdirectional sensitivity, which in conjunction with the transducer'sdirectional sensitivity, serves to limit the angular acceptance of thesystem. Thus, stray wave energy rarely causes substantial interferencein the received electronic signal.

In cases were a parasitic acoustic path has a delay time longer than thedelay times of the desired signals, the parasitic signals may beeliminated by time gating of the received signals.

The elements of the reflective array are designed to be inefficientreflectors, i.e., they allow a substantial portion of the wave to passunscattered, with a small portion being scattered according to knownprinciples. In a preferred array design, of order 1% of incident waveenergy is scattered at elements of an array. Since the reflectiveelements each reflect of order 1% of the incident wave energy, reflectedwaves which are not directed directly toward the receiving transducerwill require an additional reflection, and therefore will besubstantially attenuated as compared to a desired signal. Studies by thepresent inventor have determined, in fact, that the primary interferencedue to stray acoustic energy is related to parasitic paths having asmall number of reflections in its wave path, rather than scattered waveenergy having elongated paths involving multiple reflections. Thus,interference may be controlled by attention to a relatively small numberof direct parasitic paths rather than a large number of indirect paths.

Another result believed attributable to the relatively low reflectivityof the reflective elements and arrays as a whole is that small signalpresumptions, e.g. the principle of superposition, are valid whenanalyzing the reflective arrays. Thus, the present inventor has foundthat low reflectivity reflective arrays, when overlayed, superposegenerally linearly, without significant higher order effects. Thus, theintersections of the overlayed elements, as well as potentialresonances, did not result in artifacts, distortion, undue parasiticpaths or inoperability.

Signal Equalization Methods

Controller electronics and associated touch recognition algorithmstypically can accommodate variations in signal amplitudes (before atouch) without variation in the touch sensitivity as perceived by auser. There is always a limit to the dynamic range of amplitudes thatcan be thus accommodated. Hence, it is generally desirable to limit thedynamic range of signal amplitudes within the useful time window of areceived signal for the set of acoustic paths of a sensor subsystem. Thevariation is signal amplitudes can be controlled by a number of signalequalization methods.

Below a number of signal equalization methods are described.Combinations of these methods may be used simultaneously.

The power density of reflected acoustic waves may be controlled by a"reflective element withdrawal" method, as discussed in U.S. Pat. No.4,644,100 and Re. 33,151. Here the spacing of reflector elements isvaried. Here the "n" in the spacing vector formula, and hence the seriesof discrete options for the spacing vector S compatible with desiredincident and reflected wave vectors, is used advantageously as a locallyvarying array parameter. The "reflective element withdrawal" method issuch that selected reflective elements in the array are eliminated froma nominal dense n=1 array. It is noted that for superposed or dual usearrays, the optimal reflective element placement may vary from thatprovided for simple reflective arrays.

Note that with the withdrawal method, the local spacing vector isincreased in length by an integral multiple, e.g., m. This increase inspacing makes it more likely that coherent parasitic scattering willtake place because withdrawn reflectors are no longer available toprovide destructive interference for undesired scattering. Moreformally, an integer value for n' may exist for a parasitic scatteredwave vector k'_(R) for a value of m greater than one, where no suchsolution exists for m=1:

    mS(s)=2πn'(k.sub.I -k'.sub.R (s))/[(k.sub.I -k'.sub.R (s))·(k.sub.I -k'.sub.R (s))]

In cases where this proves to be a significant cause of parasiticsignals, alternate signal equalization techniques are preferred.

Another method of obtaining constant power density includes varying thepower reflectivity at points along the array by providing a variableheight reflective elements. Such reflecting elements of reflectivearrays having varying height are known, see U.S. Pat. No. 4,746,914,incorporated herein by reference.

A further method of modulating reflected power is the use of segmentedor truncated reflective array elements, having interrupted reflectiveelements, e.g., dashed or dotted lines, or staggered lines. See, U.S.Pat Nos. Re. 33,151, and 4,700,176, FIGS. 9, 10 and 10a, andaccompanying text, incorporated herein by reference.

A still further method of modulating reflected power is to vary the linewidth of reflector lines. A reflector element of finite width can beanalyzed as a superposition of many adjacent reflector elements ofinfinitesimal width. The scattering amplitude for the finite widthreflector is the vector sum of the scattering amplitudes of itsinfinitesimal elements. In known sensors, the line width of thereflector element is designed so that there is a 180° path-length phasedelay between the scattering off of the first and last infinitesimalreflector elements of the line width; and hence the line width is chosento maximize the scattering amplitude. Therefore, the present inventionencompasses a new reflected power modulation technique, that ofreflective element line-width modulation. This method has an interestingbenefit in the context of the present invention; for some parasiticscatterings, it is possible to pick a line width that substantiallyeliminates the parasitic scattering amplitude and yet supports a usefulscattering amplitude for the desired signal path. This method is alsoadvantageous for superposed reflective arrays because it allowsmodulation of reflected power while maintaining a uniform fritthickness. More generally this method supports array designs suited tomanufacturing processes that support only one modified acousticimpedance of the substrate.

Segmented Reflective Arrays

In order to reduce an acoustic path length in large substrates, a sideof a substrate mat be subdivided into a plurality of segments, each withits own transducer and reflective array associated with that transducer,as shown in FIGS. 6(a) and (b) and 16(a)-(c). Thus, an acoustic wavepath need not include the length of an entire side of a substrate. Wherethe side is bisected, in a known embodiment shown in FIG. 6(a), the twotransducers may produce or receive waves traveling in antiparalleldirections along a reflective array which is provided as two reflectivearrays which each reflect an acoustic wave directly into a touchsensitive portion of a substrate. The two reflective array portionsoverlap slightly, so that the overlapping portion is sensed by parallelwaves of the same type produced by two transducers.

More generally, a reflective array may be segmented and disposed along aperimeter of an active region of a substrate. In this case, each segmentincludes a transducer at one terminus. According to this segmented arrayarrangement, the acoustic wave path length is reduced to the distanceacross the substrate plus the lengths of the transmitting and receivingarrays, which are shorter than the length of the substrate on the sidewhere the arrays are situated.

Segmented arrays 1601 may lead to blind spots 1602 in a composite systemat the junction of adjacent segments, as shown in FIG. 16(b). In somecases of sufficiently redundant sensor systems, such blind spots may beacceptable. In other cases blind spots may be avoided by arranging thereflective arrays to be shingled, i.e., inclined and overlapping a smallamount at respective ends, as shown in FIG. 16(c), with the end of thearray 1603 associated with the transducer 1604 being placed behindanother array 1605. In this case, the angles and spacing of thereflectors should be adjusted as compared to a non-inclined array, perthe principles of FIGS. 10 and 11, to provide waves propagating alongdesired axes. In a special case noted above, as shown in FIGS. 6(a) and6(b), two end-to-end arrays 12, 13, 14, 15 (with antiparallelpropagating waves) may avoid a blind spot by providing an overlappingportion 16, 17, but this solution is only applicable to front-to-endbisected systems.

Reverse Reflection

In some cases, it may be advantageous for an array to direct an acousticpath away from the touch region towards a reflective boundary, and henceonly indirectly couple to acoustic path segments across the desiredtouch region. U.S. Pat. No. 5,260,521, incorporated herein by reference,in particular FIG. 17 and accompanying text thereof, illustrates such anarrangement. As shown in this prior art example, such reverse reflectionprovides additional opportunities to include mode-selective filteringwithin the acoustic path. It is noted that, according to the presentinvention, the reflective boundary according to the present inventionneed not comprise an edge of a substrate or be parallel to an edge ofthe substrate and/or a reflective array.

It the context of the present invention, reverse reflection may offeranother possible benefit. If a desired spacing vector of a superposed aarray

    S=2πn(k.sub.I -k.sub.R)/|k.sub.I -k.sub.R |.sup.2

contributes to an undesired parasitic acoustic path, reverse reflectioncan allow use of a similar set of acoustic wave paths in the desiredtouch sensitive region of the sensor while employing a completelydifferent direction for the spacing vector, due to a reversal of for thescattered wave vector. This adds to the design options available tosuppress parasitic paths.

The reflective portion may be a cut edge of the substrate, or one ormore parallel reflective elements providing a highly reflectiveinterface for the desired wave(s). These reflective elements may beformed in similar manner to the reflective elements of the reflectivearrays, although for increased reflectivity, a relatively largescattering strength is preferred. Advantageously, a reflective structuremay be provided which controls an angle of reflection, thereby reducingreliance on the edge condition of the glass, and allowing fine controlover the propagation angle of each wave path.

Reverse reflection typically adds to the acoustic path length and henceadds to the delay times. In some cases, this may provide means to avoidtime overlap between signals from two sensor subsystems.

Under certain circumstances it may be desired to alter a wave mode whilereflecting from a reflecting portion. In this case, the reflectiveportion may be provided as a series of parallel elements whose spacingsatisfies the vector-spacing formula for the desired scattering ofincident wave energy into the selected mode.

In various sensor configurations, the desired touch sensitive portion ofa substrate may be disposed as desired, for example medial or lateral toa reflective array with respect to a boundary of the substrate.

Electronics Transducer Interface

The transmitting and receiving transducers couple electrical energy toand from the controller to acoustic energy in the touchscreen. Whileother types of transducers are possible, transducers based onpiezoelectric elements are generally preferred for reasons of cost,mechanical compactness, and performance.

A piezoelectric element is typically in the form of a thin rectangularslab having conductive portions serving as electrodes on two opposingsurfaces. When an oscillating voltage signal is applied to theelectrodes, the resulting electric field within the piezoelectricmaterial, via the piezoelectric effect, causes the element to vibrate.Conversely, if the element is subjected to mechanical oscillations, anoscillating voltage will appear on the electrodes.

There are several options regarding the mode of the piezoelectricelement's mechanical oscillations. A common choice is the lowest-ordercompression-expansion illation with respect to the thin dimension of theelement; such an element couples to bulk pressure waves or otheracoustic modes with a significant longitudinal component. Another optionis a lowest-order shear oscillation in which one electrode-bearingsurface moves anti-parallel to the opposite face; such an elementcouples to bulk shear waves and other acoustic modes with shearcomponents. The direction of shear motion can be designed to be anydirection within the plane of the electrodes. More complex options arealso possible. According to one aspect of the present invention, varioussets of sensing waves propagating in the substrate may be distinguishedaccording to their propagation mode by selective coupling to appropriatemode-sensitive transducers.

Typically, piezoelectric elements are designed to have a resonantfrequency at the operating frequency for the desired mode ofoscillation. For lowest order compression oscillation, the resonantfrequency is the bulk pressure-wave velocity (in the piezoelectricmaterial) divided by twice the piezoelectric element thickness so thatthe piezo transducer element is a half wavelength thick. Similarly, alowest-order shear-mode piezoelectric element is half of a bulk-shearwavelength thick. As used in a touchscreen, the piezoelectric element isa damped mechanical oscillator due to coupling to acoustic waves in thesubstrate. The mechanical quality factor, Q, is typically in the rangefrom 5 to 20. Viewed as a frequency filter, the piezoelectric elementhas a corresponding relativcly broad bandwidth; two signals atfrequencies that are considered distinct by controller receivercircuitry might still use a common transducer.

A piezoelectric element may be bonded directly to the touchscreensubstrate and thus form a transducer. For example, see FIG. 2B of U.S.Pat. No. 5,162,618, incorporated herein by reference in its entirety.Such transducers are typically placed on the edge of the substrate andreferred to as "edge transducers". A transducer may include more thanone piezoelectric element which may help coupling to the desiredacoustic mode; for example see FIG. 2D of U.S. Pat. No. 5,162,618. Alltransducers shown in FIG. 2 of 5,162,618 may be used with thisinvention.

The piezoelectric element may be coupled indirectly to the touchscreensubstrate. For example, see the "flexible connector" that serves as anacoustic transmission line between the piezoelectric element and thesubstrate in FIG. 12 of U.S. Pat. No. 5,072,427, incorporated herein byreference in its entirety.

In a preferred embodiment, the transducer includes a wedge shapedcoupling block between the piezoelectric element and the touchsubstrate. When used as a transmitting transducer, the piezoelectricelement generates bulk waves in the wedge material which in turn coupleto the desired acoustic mode in the touch substrate. For example, incommercial touchscreen products of Elo TouchSystems, pressure-modepiezoelectric elements are coupled to Rayleigh waves in this fashionwith an acrylic wedge. Alternatively, a wedge transducer may couple ahorizontally polarized shear piezoelectric element to Love waves in asuitable touch substrate. The wedge material must have a bulk waveacoustic velocity that is slower than the phase velocity of the desiredmode in the touch substrate; the cosine of the wedge angle equals theratio of these two velocities.

The receiving transducer may also serve as the transmitting transducerin certain embodiments. When the same transducer is used for bothtransmission and reception, the low-voltage high-sensitivity receiverelectronics may be temporarily disconnected through a high impedanceswitch from the higher voltage transmitting electronics.

The transmitting transducer receives a sine wave or pseudo sine wavetone burst at the desired frequency, from the controller. This bursttypically has a power spectrum with a maximum at a nominal operatingfrequency. Normally, the sensor is tuned for use at a specific frequencyor set of frequencies, and therefore this parameter is predetermined.See, U.S. Pat. No. 4,644,100, Re. 33,151, and 4,700,176, incorporatedherein by reference.

Piezoelectric transducers of the type described are inherentlydirectional. Acoustic-electronic coupling is strongest when all parts ofa piezoelectric are driven, electronically or mechanically, in phase.Thus, waves which are incident at a skewed angle to the face of thepiezoelectric element, have a substantially reduced off axis response.

Typically, transducers are mode selective. For example, a wedgetransducer with a pressure-mode piezoelectric element may be sensitiveto Rayleigh waves but insensitive to horizontally polarized shear waves.A transducer based on an edge mounted horizontally-polarized shear modepiezoelectric element may be sensitive to ZOHPS waves but insensitive toLamb waves. The mode selectivity of transducers contribute to thesuppression of signals from parasitic acoustic paths.

It is noted that, as used herein, the transducer system comprises thetransducer and any associated array. Therefore, the transducer systemgenerates sets of incrementally varying waves traveling through thesubstrate, which may be of the same or different wave propagation modeas generated by the transducer itself.

Control System

The control system has a number of functions. First, an electronicsignal is generated, which excites the transducer to generate anacoustic waves which subsequently form the sets of waves. A transducerthen receives the sets of waves, and transduces them to an electricalsignal. The electrical signal is received, retaining significantinformation with a relatively high data rate in a low level controlsystem. An intermediate level control system, often combinedstructurally with the low level control, processes the received data,seeking to identify and characterize perturbations. For example, in oneembodiment, the intermediate level control filters the signal, performsbaseline correction, and determines a relation of the signal to athreshold. A high level control analyzes the signal perturbations andoutputs a touch position. The control system as a whole therefore hasthe functions of exciting an acoustic wave, receiving portions of theacoustic wave bearing touch information as a perturbation, and analyzingthe received portions to extract characteristics of the touch, e.g.,position.

In a preferred embodiment, as discussed in detail below, the electronicsignal exciting the transmitting transducer is in the form of a shorttone burst and delay times are determined for perturbations in thereceived signals. Alternately, the control system may determine whichwave paths are absorbed by a touch via a frequency analysis of signalperturbations. See, e.g., U.S. Pat. No. 4,645,870.

A. Implementations

A typical control system for a touch position sensor includes a digitalmicrocomputer system having program instructions stored in anon-volatile memory. This, for example, is an 8 or 16 bitmicrocontroller having internal CPU, RAM, counters and timers andpossibly other functionality. Thus, an industry standard 80C51derivative microcomputer or an ASIC device including an 80C51 core maybe used. Likewise, a digital signal processor may be employed foranalysis of the waveforms, or a low cost RISC microcontroller such asthe Microchip PIC 16X and 17X RISC microcontrollers may also beemployed.

The circuitry to implement the control according to the presentinvention may be provided as discrete devices, standard devicespartitioned according to general availability, or as highly integratedcircuits such as application specific integrated circuits (ASICs). Apreferred control circuit is provided as a pair of ASICs which aregenerally involved in digital control of burst and acquisition cycles,and analog transmit and receive functions of the touchscreen systems,respectively. Traditionally, the excitation necessary for piezoelectrictransducers in acoustic touchscreens has required relatively highvoltages in the tens of volts range, and therefore discretesemiconductor devices were employed for this function. However,according to several embodiments of the present invention, a logic leveltransducer excitation is possible, permitting highly integrated mixedsignal ASICs to implement the control. Therefore, the present inventionalso encompasses a highly integrated control circuit to implement theexcitation and receive functions, and optionally the programmablemicrocontroller. Logic level voltages are intended to mean signalsintended for communication between digital integrated circuits.Therefore, one advantageous embodiment may include excitation usingvoltages intended for intercomputer device communication, which maytypically have higher voltages than intracomputer communicationsdevices. Thus, such voltages may be derived from an RS-232communications circuit, such as may be used to communicate touchposition output.

While the various functions of the control are described hereinseparately, it should be understood that in many instances, a high levelof functional integration is preferred, and therefore it is understoodthat common hardware elements may be used for the various functions.However, in some instances, especially where the touch sensing systemsare employed with host computers, some of the high level functions maybe implemented on the host computer as a program or so-called devicedriver.

B. Excitation

The excitation function is generally straightforward. A series of pulsesor shaped pulses are emitted in a defined pattern, having substantialpower spectrum density at a nominal operating frequency of frequencies.Because this pulse is of limited duration, it has a finite band width.For example, Elo TouchSystems manufactures a controller which can excite5.53 MHz tone bursts with durations in the range of 6 to 42oscillations. This electronic pulse train drives a transmit transducerswhich emits acoustic waves traveling away from the transmittingtransducer. The wave is highly directional, and travels along an axis,which for example passes through a reflective array.

Where high flexibility of control over the excitation burst is desired,a direct digital synthesizer, such as the Analog Devices AD9850 may beemployed.

During the excitation, it is generally desired that the receivingcircuitry, which may be multiplexed to receive signals from a pluralityof transducers, be electrically isolated, e.g., through electronicswitches and/or diodes, so that parasitic electrical paths from theexcitation circuitry do not overload the high gain receiver circuitry orinfluence charge storage elements including filters. When the excitationpulse is finished, the receiving circuitry is then connected to thereceiving transducer, which may be the same as the transmittingtransducer, by electronic switches. The isolation circuitry maycooperate with the multiplexing circuitry. The excitation circuitry maybe adaptive to accommodate variations in the characteristics of thesensor subsystems. For example, Elo TouchSystem controller productE281-2310 adapts to touchscreens of varying signal attenuations byadjusting the burst pulse duration.

C. Bandwith Considerations

The excitation tone burst is of finite duration and will hence include anumber of frequency components including the nominal transmit frequency.This frequency spread is typically broad compared to the relativelynarrow frequency filtering characteristics of the reflective arrays.

The reflective arrays act as filters, and are generally the narrowestband filtering system of the transducer. For well equalized signalamplitudes and for high quality array design and manufacturing controlof reflector spacings, the band width can approach the theoretical limitproportional to the inverse of the signal duration. Thus, much of thebroadband energy from the excitation pulse is not coherently scatteredor is misdirected by the reflective arrays and not received with thedesired signal(s). In some cases, components of the broadband waveenergy may lead to parasitic signals due to sidelobes, mode conversion,or other undesired effects which allow this wave energy to interferewith receipt of the desired waves. This provides a motivation to limitthe bandwidth of the receiving circuitry. Thus, the receiver may includea selective filter.

From a frequency-domain perspective, a touch alters the frequencyfiltering characteristics of the sensor subsystem. A perturbation due toa touch which is narrow in the time domain will lead, via thefundamental mathematical relationships between bandwidth and timeduration, to a broadening of the frequency filter characteristics of thesensor subsystem. The transmitted tone bursts are generally kept shortenough so that the sensor subsystem is excited for the band offrequencies containing the touch information. The bandwidth of thebandpass filters in the receiving circuitry must have sufficient widthto pass frequency components containing the touch information.

D. Receiver

The control receives the signals corresponding to the acoustic wavesfrom the transducer and processes the signals for analysis. The controlretains relevant data and may, and indeed preferably, filters extraneousdata from the signal. Thus, where the relevant information is time andamplitude for a smoothed wave-form, other information may be ignoredsuch as phase, frequency components outside the desired bandwidth, andsignals for receive transducers for other sensor subsystems. Further,the type of data contained in the signal defines the simplest acceptablecontroller configuration. However, other types of information may alsobe included within the signal. Analysis of this information may beuseful in analyzing the signal. The purpose of the receiver is toextract information from an acoustic wave signal which may represent itssource, path, characteristics or type of perturbation(s), timing ofperturbation(s), duration of perturbation(s), frequency characteristicsof perturbation(s), pressure or amplitude of perturbation(s), andinterferences or artifacts. Thus, the receiver takes a signal having alarge amount of raw data and produces vectors representing significantfeatures of the signal.

According to certain embodiments of the present invention, differenttypes of waves may be time multiplexed, e.g., applied to the substratesequentially, and therefore need not be present or analyzedsimultaneously. Thus, where excitation of transmitted waves havingdiffering frequencies or wavelengths may be independently selected,these may be time multiplexed onto the substrate. Therefore, a receivermay also be multiplexed to operate in a plurality of states.Alternately, various wave modes may be simultaneously applied to thesubstrate and resolved by selecting one of two or more receivetransducers receiving signals from a common transmit transducer, withselective processing at the receiving transducer or in the receivingelectronics.

The receive circuitry may be adaptive to variations in the physicalcharacteristics of the sensor subsystems. For example, Elo TouchSystemcontroller product E281-2310 adapts to sensor subsystems of varyingsizes by adjusting time windows used to gate received signals.

A multivariate analysis of different parameters may be employed toobtain further information from the received data. If more than one wavetype is available, a controller may select an optimal wave or set ofwaves to analyze the sensed variable, i.e., under various conditions, asubset of the available information may be analyzed or employed indetermining the output.

In many embodiments, it is not necessary to capture phase informationcontained in the received signals. However, in some instances, it may beadvantageous, or even essential to do so. As discussed below, use ofphase information allows generalization to cases in which a desiredsignal is coherently superposed on another desired signal or a parasiticsignal. This novel feature of this invention is discussed in more detailbelow.

Phase and Separation of Superposed Signals

The phase information in received signals may be used to disentanglesignals overlapping in time. With reference to FIG. 29(a), the effect ofsuperposed waves is shown. In a phase sensitive receiver system, aperturbation is detected at an i^(th) time slice if the magnitude ofsignal vector S_(i) is significantly larger or smaller than themagnitude of the reference vector R_(i), or if the relative phase shift

    Δφ=φ.sub.i -(φ.sub.i+.sbsb.Δ =φ.sub.i-.sbsb.Δ)/2

is significantly non-zero, or some weighted combination of both such asthe quadratic sum of Δφ and |S_(i) -R_(i) |/|R_(i) | is greater than athreshold. In this formula, Δ is an integer comparable or larger thanthe number of time slices occupied by a typical touch perturbation. Notethat the value of Δφ, and hence the above conditions, are not affectedby global drifts in absolute phase, nor by global changes in the slopeof phase with time. Thus, a phase sensitive receiver system can reliablydetect a perturbation of a desired signal even in the presence of aninterfering signal. On the other hand, even if a significant change inthe signal phase at the i^(th) time slice occurs, this may be ignored byan AM detection system if |S_(i) |≈|R_(i) |.

This threshold need not be a global value, and may therefore varybetween different regions of the sensor. In theory, the threshold is setto reliably detect touches (low false negative rate) while preventingoutputs indicative of touch which do not correspond to a real touch (lowfalse positive rate). This may be optimized, for example, by determininga noise or signal instability level in a given region, and setting thethreshold above an average noise or instability level for that region.The threshold may be redetermined periodically, continuously, and/orbased on the occurrence of an event. While possibly more pertinent for aphase sensitive controller, a regionally determined threshold system mayalso be applied to traditional AM detector systems. The regions maycorrespond to the time dimension of a received signal, a physicalcoordinate of the sensor system, or other convenient space. Thethreshold determination may occur before, after or in conjunction with alogical analysis of the signal(s).

In a phase sensitive system, to ensure the perturbation is localized asexpected for a touch, a further analysis may be conducted, requiring thefollowing conditions on the signal amplitudes for the time slices asuitably small number of steps, Δ, away from the i^(th) time slice.

    φ.sub.i+.sbsb.Δ ≈φ.sub.i-.sbsb.Δ |S.sub.i+.sbsb.Δ |≈|R.sub.i+.sbsb.Δ ||S.sub.i-.sbsb.Δ |≈|R.sub.i-.sbsb.Δ |

Further refinements and elaborations of these basic phase-algorithmprinciples can be supported by modern electronics. For example, a DSPfilter design may be provided, having adaptive capabilities, i.e., itmay learn new compensation strategies or details and apply these asnecessary. It is noted that DSP functionality may be implemented as adedicated semiconductor design, e.g., a DSP or digital filter, or may beprovided as software controlled functionality of a general purposeprocessor. Suitable DSP devices include TMS320C2x, C3X or C5X devicesfrom Texas Instruments, MC56000 series DSPs from Motorola, Z8 seriesmicrocontrollers including DSP capability from Zilog, etc. DSPfunctionality may also be obtained through application specificintegrated circuits, programmable logic devices, and other types ofsemiconductor devices.

The acoustic signal propagation time for is very short as compared tohuman reaction times. Therefore, the signal need not be fully processedin real time. Typically it is sufficient to simply capture and digitizea wave-form information as it is received. The captured raw data can beprocessed later. It is possible to sequentially excite a plurality ofsensor subsystems and process of signal data from one subsystem whileanother is excited. Thus, where a control system is provided which mayanalyze the received signals, the substrate is available for additionalmeasurement cycles, allowing various wave mode excitation cycles to beanalyzed independently. Under circumstances where a high speed of dataacquisition is desired, or computational power is available for thispurpose, multiple sensor subsystems may also be analyzed in parallel.

D(i). AM Detection

For embodiments in which signal amplitude information is sufficient toreconstruct touch positions, an AM (amplitude modulation) detectioncircuitry may be used. This is likely to be the case if interferencefrom parasitic acoustic paths is negligible and if there is neversimultaneous receipt of more than one desired signal, from more than onesensor subsystem, at a receiving transducer. Prior art acoustic touchsystems assume a requirement the received signals can be successfullyprocessed via AM detection. For the present invention, this is not arequirement, but will nevertheless be true for many embodiments. Atypical AM detection circuit is shown in FIG. 30.

AM detection methods have been widely used in the prior art. Thus, AMreceivers are disclosed in U.S. Pat. Nos. 5,162,618, 5,072,427,5,177,327, 5,243,148 and 5,329,070, expressly incorporated herein byreference and in U.S. Pat. Nos. 4,642,423, 4,700,176, 4,644,100 and Re.33,151, expressly incorporated herein by reference, which may beemployed, as applicable, with sensor embodiments. Controllers foracoustic touch panels are also disclosed in U.S. Pat. Nos. 5,380,959,and 5,334,805, expressly incorporated herein by reference.

The following AM detection scheme is typical of present commercialcontroller products. After a pre-amplifier stage, one for each receivetransducer, the received signal is multiplexed to, e.g. an MC1350 RFautomatic gain controlled amplifier. The signal is then detected, by forexample a full or half wave rectifier circuit, a synchronous rectifieror an MC1330 detector. The detector is generally followed by a singlepole low pass filter. Bandwidth limiting is applied at various stages ofthe signal chain. The resulting bandwidth is typically less than 0.5MHz. The rectified signal, smoothed due to bandwidth limiting, is thenbuffered and digitized by an 8 or 12 bit analog-digital converter. Onedigitized sample per microsecond is typical. The digitized data may beanalyzed in real time and/or buffered for later analysis.

Prior art approaches to AM detection of touchscreen signals are not theonly AM detection techniques available. For example, the receivercircuit may include a tuned narrow band AM superheterodyne receiver. Thereceived signal, with a known transmission frequency, is initiallyamplified in a low noise, high gain video-type amplifier. The amplifiedcarrier and signal is then mixed with an AFC (automatic frequencycontrol) tuning signal to achieve an IF of, for example, 3.54 MHz. Thenarrow band IF (intermediate frequency) signal is filtered to eliminateother frequencies but retain amplitude information having a bandwidth ofless than 500 kHz, and then amplified. The amplified, filtered IF signalis then detected by a full wave rectification and filtering orsynchronous detection. The AM detected signal is digitized.

For many embodiments of the present invention, the low-level recognitionof signal perturbations may proceed via known AM detection schemes,while the higher-level logical analysis of identified signalperturbations differ from known systems, as described herein.

A variation of AM detection methods disclosed in U.S. Pat. Nos.5,380,959, and 5,334,805, in which distinct X and Y receive transducersgenerate signals. One receiver responds to the desired signal plusinterference from acoustic parasitic paths or electromagneticbackgrounds, and the other responds only to the interference. Whileperhaps atypical, there may be cases in which the interference isreceived with the same phase and amplitude for both the X and Y receivetransducers. In such cases the interference-only signal may becoherently subtracted from the other signal, and the resultinginterference-canceled signal processed via standard AM detectionmethods. This combining of signals from two receive transducers is verydifferent from the phase-sensitive detection methods described below, inwhich a controller can process input from a single receive transducereven if multiple signals are superposed; the apparent purpose of thedual receiving transducer circuit of U.S. Pat. Nos. 5,380,959 and5,334,805 is to avoid the need to for processing of signals overlappingin time that would otherwise interfere with AM detection schemes.

As shown in FIG. 30, a pair of transducers, X and Y, provide inputs toband pass filters 3001 and 3002. These band pass filters may also benotch filters, and indeed the preamplifiers 3003, 3004 are bandlimiting. The outputs of the preamplifiers 3003, 3004 are multiplexedthrough multiplexer 3005, depending on which transducer is activelyreceiving signal. A further bandpass filter 3006 may be provided. Avoltage controlled amplifier 3007 is provided to allow a controller toadjust the gain of the input channel. A band pass filter 3008 eliminatesDC and low frequency components, as well as high frequency noise, andthe amplitude of the signal is detected in a full wave rectifier 3009 orRMS circuit. The detected output is then again filtered with a low passfilter 3010, amplified with amplifier 3011, and output. The output maybe fed, for example, to a sample and hold amplifier and analog todigital converter (not shown).

D(ii). Phase Sensitive Detection

According to one set of embodiments according to the present invention,two (or more) sets of wave paths are superposed and portions arereceived simultaneously at the receiver. This scenario is demonstratedin FIG. 9D(1). A superposed set of wave paths may correspond to adesired sensor subsystem, or be due to parasitic acoustic paths. Nopresumption is made that there is any particular absolute phaserelationship between the various sets of superposed wave paths,furthermore the phase relationships between superposed signals may driftwith time.

Four representative relative phase states of two waves are: in phase,out of phase, leading and lagging. For example, waves having equalamplitude pass through a portion of a substrate subject to a touchinduced perturbation, where, for illustrative purposes, we assume afinger touch completely absorbs one of the waves. If the waves are inphase, the touch will result in a factor of two reduction in totalamplitude. If the waves have leading and lagging phase relationship of90°, the finger touch results in only a 29% reduction in the magnitudeof the net signal amplitude. If the waves are out of phase (180° apart),the net amplitude will be zero before the touch, and a finite signalwill appear due to the finger touch. Such effects confuse typical touchrecognition algorithms based on AM detection schemes, and hence is whyit has been heretofore been considered unacceptable for sensor design toallow signal amplitudes to combine in such a fashion.

As shown in FIG. 9D(1), two signals, each bearing touch information, arepresent simultaneously. When summed, there are a number ofpossibilities, as discussed above. FIG. 9D(2) shows a phase coherentsuperposition, wherein the amplitudes of the two waves are additive. Ina phase coherent detection scheme, as shown in FIG. 9D(3), the secondwaveform 21 is subtracted from the first waveform 20, with thepossibility of negative amplitudes 22, and an increase in signalamplitude corresponding to a wave perturbation 23. FIG. 9D(4) shows adestructive interference of the two waves, with RMS detection of theresulting waveform, so that negative amplitudes are not possible. Moregenerally, the phase relationship between the two signals in FIG. 9D(1)may drift while they are being received, thus leading to signals thatare even more problematic for AM detection schemes.

Embodiments in which the controller captures phase as well as amplitudeinformation can utilize such superposed signals. For example, as notedin FIG. 29(a), signal perturbations due to touches may be recognized bya displacement of the net amplitude in two-dimensional I-Q space. Thisis true even if the magnitude of the net signal does not change. Even inthe presence of superposed signals, acoustic attenuation of the desiredsignal will always change the net signal vector in I-Q space. As notedin the discussion above regarding FIG. 29(a), it is possible toimplement algorithms that filter our various drifts in the global phasesof the superposed sets of waves.

In cases where the drifts in the global phases of superposed sets ofwaves is slow compared to the time it takes the controller to update thereference I-Q amplitude vectors R_(i), the presence of a touch may besimply recognized as a significantly non-zero value for |S_(i) -R_(i) |.

There are many ways to implement a phase-sensitive controller. One basicapproach is to compare the phase of received signals with a free runningreference clock signal, as shown in FIG. 31(c). Other approaches maymake use of phase-locked loops, as shown in FIGS. 31(a), 31(b), and31(d).

In the context of FIG. 31(c), homodyne mixing provides a particularlysimple conceptual approach. For example, for a sensor operating at 5MHz, a continuous 5 MHz reference clock signal 3101 is generated. Thetone burst to the transmit transducer has a fixed phase relationship tothis reference clock signal. An "In-phase" or "I" copy of a referenceclock signal 3102, derived by time-shifting the clock with phase shifter3103, as necessary, is mixed in mixer 3104 with a filtered signal 3106from a receive transducer 3105, and the resulting mixed signal passedthrough a low-pass filter 3107 and then digitized with analog to digitalconverter 3108; thus an I wave-form 3109 is captured. A "Quadrature" or"Q" copy of the reference clock signal is generated with a 90° phaseshift with respect to the I reference clock signal, generated byquadrature generator 3110; this is similarly used to capture the Qwave-form 3114 through a mixer 3111, low pass filter 3112 and analog todigital converter 3113. In this fashion, S_(i) =(I_(i),Q_(i)) isdirectly generated for use in the implementation of algorithms based onFIG. 29(a). The digital processing may be accomplished with generalpurpose microprocessor, or more specialized digital signal processors,not shown in FIG. 31(c).

An alternate phase locked loop embodiment is shown in FIG. 31(a). Inthis figure, the transmit burst signal generator also controls thereceive clock generator. The transmit burst generator 3147 produces anoutput which is influenced by a model 3148, which is, for example, a setof delays and filters. This is then fed to the phase locked loop circuit3149. The remainder of FIG. 31(a) is similar to the circuit of FIG.31(b), discussed below.

In certain instances, it may be preferable to capture the receivedsignal from the receive transducer 3122 with sufficient high timeresolution, e.g., four times the carrier frequency f₀, to capture the RFwaveform. The received signal is preferably filtered with a narrow bandfilter 3127. In this case, the sampling clock 3123 provides a phasereference, to operate a track and hold amplifier 3124 and analog todigital converter 3125, as shown in FIGS. 31(e) and 31(f). Inparticular, a system as shown in FIG. 31(e) provides the fullflexibility provided by programmable digital signal processing 3126. Forexample, in cases where two or more operating frequencies are used, aparticular received signal may be selected, in part, by loadingappropriate digital-signal-processing constants needed to produce afilter at the desired frequency selected the in software. FIG. 31(f)provides an application specific digital signal processing circuit 3120,which performs phase extraction, digital filtering, and decimation intime. Thus, the output data rate is reduced, and a typical microcomputer3121, without particular digital signal processing prowess, may be usedto perform further analysis.

In such a system that captures a RF wave-form, the received signal isdigitized directly at a rate in excess of the Nyquist rate, afterpreamplification and standard signal conditioning (which may includenarrow band filtering), and is then processed using a digital signalprocessor (DSP) 3126. In this case, the DSP 3126 may operate in realtime, or buffer the received digitized waveform in a RAM and process itwith some latency. For example, with a 5.53 MHz excitation, and a 500 μSecho analysis, a RAM buffer on the order of about 16 kWords may berequired, with samples acquired about every 40 nanoseconds (25Megasamples per second). Of course, this storage requirement may bereduced if the entire signal need not be fully analyzed at one time; forexample, the signal may be divided timewise, and blocks of consecutivesamples analyzed consecutively. This will focus the analysis on sectionsof the sensor for each excitation burst. The actual sample timing mayvary adaptively to track the received waveform or be at a constant rate.The digitized signal, after detection of the relevant parameters, maythen be digitally filtered (FIR, IIR, auto regression, or more complexfilters such as auto regression and moving average process filtering),waveform analyzed, adaptively compensated, compared to a reference, andsubject to other techniques to determine waveform perturbingcharacteristics of the touch, such as location, z-axis (pressure), andtype of object (wave mode absorption characteristics). Thus, aselective, sensitive system is provided.

The reference clock signal may be generated from the received signalwith a phase-locked loop 3130, as shown in FIG. 31(d). Typically, aphase locked loop 3130 may be implemented in hardware, so that anoscillating signal is generated which corresponds to a "carrier" wave ofanother signal. As shown in more detail in FIG. 31(b), the phase lockedloop 3131 tracks the signal, but is limited to vary more slowly by timedelays 3132. Thus, drift in phase due to manufacturing tolerances andenvironmental effects may be filtered, and yet any rapid changes inphase due to a touch may be determined by comparing the generatedcarrier with the actual signal, or by analyzing the error signal. Alsoshown in FIG. 31(b) is a pair of mixers 3133, 3134, for mixing the inphase and quadrature synthesized clock outputs 3135 with the receivedsignal, and a pair of RMS detectors 3136, 3137 to detect each phasesignal. As shown, the circuit seeks to maximize the difference betweenthe RMS value of the I and Q signals, to shift the phase of the VCO 3138when a relative change occurs in the respective amplitudes. The VCO 3138has a lock input 3139 from the controller 3140 to prevent compensatorychanges, for example where a touch is detected. The RMS values of the Iand Q signals are multiplexed in multiplexer 3141, and digitized inanalog to digital converter 3142, and input to the controller 3140. Thecontroller 3140 has associated RAM 3143, for storing transient data, andROM 3144, for storing programs and tables, as well as input and outputdrivers 3145. As shown in FIG. 31(d), the detection circuit 3146 maygenerate the I and Q signals, which are multiplexed and digitized,without RMS processing.

If the control feedback of a phased-locked loop is sufficient fast,then, for example, the voltage controlling a variable-frequencyoscillator within the phased-locked loop may be integrated and digitizedto directly provide a measure of signal phase. With a DC-reject filter,variations in the global phase offsets can be eliminated. Combined withAM detection, this provides digitized I-Q signal amplitudes in polarcoordinates. Thus phase-sensitive controllers may digitize signals inI-Q space in either Cartesian or polar coordinates.

The electronics industry provides, and can be expected to continue tointroduce, components that enable fabrication of phase-sensitivecontrollers. For example, impressive digital signal processing power canbe provided by an Intel Pentium processor or a Texas Instruments TMS320C80 DSP coprocessor. Rapid digitization of signals is possible at 40MSPS from one or more Texas Instruments TLC5540INSLE 8-bit ADCs, or at10-bit resolution at 20 MSPS from one or more Analog Devices ADS820 10bit ADCs. Suitable PLLs are, for example, the Signetics NE/SE564, NE568,(or equivalents) and Texas Instruments TLC2932 (or equivalent). Suitablemixers include the NE/SA602 and NE612 (or equivalent), which eachinclude a double-balanced mixer circuit. See, Signetics NE/SA602 datasheet and Signetics AN 1981 and AD1982. The cost-performance trade-offsfor particular applications will determine the most suitable choice ofcomponents for phase-sensitive controllers.

Use of intermediate frequencies ("IF") with heterodyne mixing may beconsidered as a means to shift received signals to a frequency which isa standard for a mass-market signal processing application. For example455 kHz and 10.7 MHz are standard IF frequencies for radio and videocommunication. Further, where a plurality of frequencies are to bereceived, a heterodyne receiver allows a common filter and receiver tobe employed for receiving the various signals after tuning. Note that itis possible for the intermediate frequency to be higher than theoperating frequency for the touchscreens. The use of IF frequenciesfurther increases options for electronic components to be used asbuilding blocks of a phase-sensitive controller.

E. Intermediate Level Processing

The purpose of intermediate level processing is to efficientlyreconstruct delay-times, and perhaps quantitative absorptioninformation, for perturbations of the signals due to touches, and to doso with sufficient immunity to signal artifacts due to temperature,humidity, electronic emissions, radio frequency interference, and thelike. Intermediate-level processing need not be fool-proof, as higherorder processing within the algorithm may also reject artifacts. Iffact, intermediate level processing can be optimized for efficiency atthe expense of immunity if the higher level algorithms requireconsistent and redundant information from three or more sensorsubsystems.

Typically, the relevant touch information is contained within arelatively narrow bandwidth about the nominal operating frequency orfrequencies. Many artifacts can be eliminated with appropriate frequencyfiltering, either in hardware or via digital signal processing.

Touch perturbations occur on a fast time scale relative to many sourcesof signal drift. Therefore, as is typical of the prior art, an adaptivebaseline is used to distinguish genuine touch information fromsystematic drifts in signal amplitudes. Note, however, in contrast toprior art, the adaptive baseline need not be limited to AM amplitudeinformation; the adaptive baseline may also incorporate phaseinformation.

As discussed above, the baseline and/or threshold processing may beregionally varying, and may be optimized for highest performance.

In the context of a phase-sensitive controller, use of an adaptivebaseline is conceptually similar to known AM systems. Thus, for example,an adaptive baseline correction is implemented by memorizing a baselinepattern and analyzing the received signal with respect to the memorizedbaseline to determine the presence of a perturbation. The referencecondition compensates for long term and environmental conditions, andfacilitates meaningful analysis of the received signals. The referencecondition is preferably derived periodically in the absence of a touchor other indications of transient conditions. Furthermore, the baselinemay also be updated continuously based on portions of the signalpresumed to be unaffected by transient conditions, even during periodswhen a touch occurs. The reference signal therefore compensates for manyphysical characteristics of the screen, contamination, as well as longterm drift due to, e.g., temperature. Known systems implement suchadaptive baselines.

Embodiments of the present invention using phase sensitive controllersnecessarily involve more than simple subtraction of a baseline. Thebaseline information, e.g. a memorized reference wave-form, containsphase as well as amplitude information. Optionally, stability or noiseinformation may be stored. Simple subtraction of reference amplitudes isreplaced by mathematical processing involving calculations ofdisplacements in I-Q space.

Whether or not the controller is phase sensitive, the received signalsare processed to provide time-delays and magnitudes of candidate touchperturbations for the sensor subsystems. This information is then usedin the next level of data processing.

In cases where there is an ambiguity regarding which sensor subsystemcorresponds to a candidate touch perturbation, all interpretations maybe provided to the higher level algorithms for further processing. Suchambiguities occur in sensor systems as shown in FIGS. 19, 22(c), and 28.A case in point is where a phase-sensitive controller simultaneouslyreceives signals from two sensor subsystems through a common receivetransducer, and the controller may not have a direct means to determinewhich of the coherently summed signals has been perturbed.

F. Analysis of Perturbation

Candidate touch perturbations are analyzed to reconstruct positions oftouches. Optionally, the touch system may output further informationregarding touches such as the touch "pressure", i.e. magnitude ofacoustic absorption, and "water-rejection" touch characteristics such asthe ratio of shear-wave to Rayleigh-wave absorption. Furthermore, thepresent invention supports algorithms with increased tolerance forshadowing due to contaminants, with unambiguous multiple-touchcapability, and with enhanced reliability due to consistency checksbased on redundant measurements.

Generalize to Non-orthogonal Sensor Subsystems

Although the scope of this invention includes schemes which output but asingle coordinate, the primary objective of most touch systems is tooutput two-dimensional coordinates of touches on a surface. If a touchis sensed by two or more sensor subsystems, then there is typically aunique touch position on the touch surface that can account for theresulting delay times for the perturbations observed in thecorresponding signals. Note that coordinates measured by the subsystemsaccording to the present invention need not correspond to an outputcoordinate axis and need not be orthogonal to each other, the arrays,nor the edges of the glass. Thus, where the sensing waves do not crossthe touch sensitive region of the substrate orthogonal to a referencecoordinate system, the controller performs a coordinate transformationon one or more received signals to the desired coordinate system. Thecoordinate transformation to the output system may occur before or afterthe two-dimensional position of the touch is reconstructed.

Where more information is received than is required for reconstructionof the touch position, e.g., three received signals for two coordinateaxes, a consistency checking and optimization analysis may be executedto make optimal use of the available information. A weighted average maybe constructed based on coordinates determined by pairs of sensorsubsystems sensing the touch. Alternately, the algorithm may use thecoordinate determined by the sensor-subsystem pair most likely toprovide reliable and accurate coordinate measurements.

Once a touch is registered, the coordinates of the touch are determinedby calculating a center of a touch, possibly with correction fornon-linearity or scaling, and output as the touch position.

There is no requirement here that there is a linear, or approximatelylinear relationship between delay times of touch perturbations and anycoordinate of interest.

Dual Mode Sensing of Touch Characteristic

As with prior art sensors, a "pressure" or "Z axis" value for the touchmay be included with the touch position as part of the output from thetouch system. Additional information may be provided by the presentinvention in cases where a touch is sensed by more than one acousticmode. See FIG. 26. For example, if a touch is sensed by both ahorizontally polarized shear wave and an acoustic mode subject toleaky-wave-radiation damping into water contaminants, such as a Rayleighwave, then the ratio of shear-wave to non-shear-wave acoustic absorptionprovides an touch characteristic that may be used for water rejection. Athreshold may be defined so that "touches" due to water drops arerejected while finger touches are accepted. This in an important featureof sensors utilizing more than one acoustic mode in the touch region.Thus, according to FIG. 26, a touch position is reconstructed 2601. Themagnitude of the signal perturbation for each wave is determined 2602.The consistency of the perturbation is tested with respect to knownconditions 2603, to allow classification of the characteristic of thetouch.

Anti-shadowing

For sensor designs in which touches are designed to be sensed with threeor more sensor subsystems, anti-shadowing algorithms are possible. See,FIGS. 25(a) and 25(b). Shadowing occurs when a contaminant or otheracoustic obstruction so reduces signal amplitudes so as to produce anunresponsive dead region. For a sensor subsystem, the dead regionincludes not only the location of the contaminant, but also the entirelength of the acoustic paths shadowed by the contaminant. For example,in rectangular sensors according to the prior art with X and Y sensorsubsystems, a strongly absorbing contaminant one inch in diameter willresult in a cross-shaped dead region, with one inch horizontal andvertical stripes intersecting at the contaminant, within whichtwo-dimensional touch coordinates cannot be reconstructed. Note that theloss of either coordinate measurement results in an inability toreconstruct a touch position. In contrast, when a touch is within thesensitive zone of three or more sensor subsystems, a two-dimensionalposition can still be determined if one coordinate is lost in the shadowof a contaminant.

FIG. 25(a) shows a simplified flow chart for an anti-shadowingalgorithm. The algorithm flow chart is abbreviated and representative,setting forth the basic steps. Application of the basic concepts herewill be considered below in the context of specific embodiments of theinvention. All significant perturbations in all sensor subsystem signalsare identified 2501. The delay times are determined for each signalperturbation 2502. Perturbations in overlapping regions of sensorsubsystems are matched 2503. Finally, from the matched sets, touchpositions are calculated 2504. It is noted that the shadowing influencewill be considered a strongly absorptive, slowly changing perturbation,in contrast to a touch, which is generally of short duration and may beless than completely absorptive. FIG. 25(b) shows an antishadowingalgorithm in more detail for an X, Y, 30° diagonal path sensor system asshown in FIG. 13. The X signal is searched for a touch 2511. If found,2512, the Y signal is searched, or if not found, the Y signal is alsosearched 2521. If X and Y are found, the touch position is reported2513. The diagonal paths of the two triangular sensor subsystems arethen searched for touch, 2514, 1517, and if found, the missing Y iscalculated 2515, 2518, and reported 2516, 2519. If no diagonal touchperturbation is found, the X touch information is likely artifact andignored 2520. If no X is found, the Y is searched for a touch 2521. Ifboth X and Y fail to show a touch, it is presumed that no touch ispresent 2522. On the other hand, if a Y touch is found, the diagonalpaths are then searched for touch 2523, 2526, and if found, the missingX calculated 2524, 2527, and reported 2525, 2528. If no diagonal touchperturbation is found, the Y touch information is likely artifact andignored 2529.

For a shadowing contaminant, particularly one which is observed by threeor more sensor subsystems, the controller has information from which onecan determine the presence and location of the contaminant. Suchinformation can also be used to provide user feedback to remedy theproblem. For example, diagnostic software may include a maintenanceoption in which "Clean me here!" messages appear as needed with arrowsand targets.

Multiple Touch Capability

A particular aspect of certain embodiments of the present invention isthe ability to detect and analyze multiple simultaneous touches, basedon the plurality of waves. See FIGS. 24(a) and 24(b).

Prior art sensors are subject to the following ambiguity whensimultaneously subjected to two or more touches. Consider an acoustictouchscreen is subjected to two touches, one may be represented withcoordinates (X₁, Y₁) and the other with coordinates (X₂,Y₂). Thereceived signal providing X coordinate information contains two signalperturbations allowing the determination of the values of X₁ and X₂. SeeFIG. 9B. Likewise the Y signal allows reconstruction of the values of Y₁and Y₂. The signals make it clear that there are two touches. However,there is an ambiguity whether the two touches are at coordinates (X₁,Y₁) and (X₂, Y₂), or alternatively at coordinates (X₁, Y₂) and (X₂, Y₁).The ambiguity concerns which X coordinate to pair up with which Ycoordinate. With increased complexity, similar ambiguities are presentfor three or more simultaneous touches.

To some extent, this ambiguity can be resolved by timing andquantitative attenuation information. If the (X₁, Y₁) touch makescontact with the sensor before the (X₂, Y₂) touch, the controller maydecide that a (X₁, Y₁)/(X₂, Y₂) double touch is more likely to follow a(X₁, Y₁) single touch than a (X₁, Y₂)/(X₂, Y₁) double touch. Similarly,touch amplitude information may be used. Assume, for example, that thesecond touch is a lighter touch, i.e. with less attenuation, than thetouch at (X₁, Y₁), being represented using lower case letters for itstouch coordinates (x₂, y₂). By matching amplitudes, the controller maydecide that a (X₁, Y₁)/(x₂, x₂) double touch is more likely than a (X₁,y₂)/(x₂,Y₁) double touch. In many cases, these methods will resolve theambiguity. However, by themselves, these methods are not alwaysreliable. Two touches may have approximately the same attenuation ormake contact simultaneously. Most importantly, if X₁ and X₂ , or Y₁ andY₂, are sufficiently close in value, the corresponding perturbations inthe signal will overlap and make it problematic to reliably andaccurately disentangle the two coordinate values.

Here, it is advantageous to cover desired touch regions with three ormore sensor subsystems, as shown in FIGS. 13 and 14. FIG. 24(b) presentsan algorithm that may be used with the sensor in FIG. 13. Using a pairof sensor subsystems, candidate touch positions, subject to ambiguities,may be determined 2401X, 2401Y. For each candidate touch position, delaytimes can be predicted for the remaining sensor subsystem(s). For thevalid touch positions, the predicted delay times will correspond tomeasured touch perturbations along all axes. More generally, a validtouch is recognized by a self-consistent triple(s) of delay times fromthree or more sensor subsystems. From these self-consistent triples, thetouch positions may be calculated.

The comparison process includes testing candidate touch positions withthe various possibilities. Thus, the candidates are identified 2405, thepredicted perturbations calculated for each candidate touch position forthe diagonal axes 2406, 2408, and the predicted perturbation comparedwith the actual perturbation for each diagonal axis 2407, 2409. Theconsistent candidates are determined for the touch positions and output2410. The candidates are evaluated successively 2411.

Also note that the touch perturbation interference problem noted here isa variant of the "contaminant" shadowing problem, and hence may besolved with the aid of anti-shadowing algorithms. Namely, the algorithmmay base touch position determination on sensor subsystem data for whichtouch perturbations are well separated.

A multiple-touch algorithm may be subject to the burden of sortingthrough a relatively large number of candidate touch positions due tothe nature of combinatorics. However, unless the number of simultaneoustouches becomes excessive, this is not a significant problem foralgorithm response times. A substantial amount of data reduction occursin the low level control, i.e., the data stream from the analog todigital converter is reduced to a relatively small set of parameters,e.g., a timing, width and amplitude of each detected perturbation.Because the amount of data to be processed by the high level control,i.e., logical analysis, is relatively small, a comprehensive analysis ispossible. Therefore, if three perturbations are detected in each ofthree sets of waves, a logical analysis of each of the 27 possiblecoordinate locations of three touches is possible. Therefore, theperturbation characteristic, attenuation and timing correlation of eachperturbation may be compared to predict a most likely set of points. Ifone of the perturbations, for example, comprises artifact orinterference, the corresponding coordinate may be excluded or ignored.

It is also noted that, in order to define a touch position from among anumber of possibilities, the resolution of ambiguity need not beperformed strictly by acoustic means. For example, a coarse infraredtouch detection grid (rotated with respect to coordinates measured byacoustic sensor subsystems) may be used to detect the gross position ofone or more touches, with the coarse position used to consistentlydetermine the exact location of multiple perturbations in the receivedacoustic wave signals.

Redundant Measurement Consistency Checks

Even when there are not discrete ambiguities due to multiple touches,algorithms based on self consistency of redundant coordinatemeasurements provides a means to eliminate artifacts. See FIG. 24(a).The use of self-consistent triples of delay times from three or moresensor subsystems goes beyond the redundancy checks used in the priorart.

According to known schemes, waves travel along axes which correspond toa pair of perpendicular physical edges or major axes of a substrate, andone set of waves senses touch along each axis. Thus, a single touch willproduce a pair of corresponding perturbations in each of the sets ofwaves, which directly correspond to the coordinate system of thesubstrate. Thus, when a touch is detected, the analysis determines theposition along each of the axes and outputs a coordinate pair. Thisknown system thus redundantly detects the presence of a touch, that isthe presence of a touch is validated if it is observed in both the X andY channels. Thus, uncorrelated noise which effects one reading may beignored if it does not effect a subsequent reading on the other axis.Further, a minimum perturbation duration may be required, so that atleast one confirming reading may be required in order for a touch toregister. However, known systems do not include a higher level ofredundancy.

The present invention, encompasses the analysis of redundancy, such thata position along one coordinate axis is determined using informationwhich provides at least two wavepaths through a touch position todetermine a position along a single axis, or two waves havingdifferential absorptivity characteristics for a touch passing throughthe touch position. According to another aspect of the invention, acoordinate transform is performed to translate signals produced bytransducers from the various wavepaths to a desired coordinate system,wherein the transform requires at least two wavepaths to define a singleoutput coordinate.

The perturbation analysis may thus also include validity checking andposition estimation based on the multiple signals, wherein aninconsistency may be present. Thus, the signals are logically analyzedaccording to rules, which may be predetermined or adaptive, to define anoutput which should be produced for a given set of inputs.

FIG. 24 shows a multiple-touch/redundancy-check algorithm flow chart fora touch sensor having two orthogonal sets of acoustic paths and anon-orthogonal set of acoustic paths. The algorithm flow chart isabbreviated and representative, setting forth the basic steps.Significant perturbations in all sensor subsystem signals are identified2401. The delay times for all perturbations is determined 2402. Selfconsistency of triples is then evaluated, with errors evaluated forsignificance 2403. The touch position(s) is then calculated 2404 andoutput. Application of the basic concepts here will be considered belowin the context of specific embodiments of the invention. Such analgorithm is able to resolve multiple-touch ambiguities, and providesfor additional noise rejection, allowing operation with a lighter touchthreshold and/or operation in a noisier environment.

Where supposedly redundant information is inconsistent, the signals maybe analyzed to determine if one of the signals is likely inaccurate orin error, and a most likely coordinate output. Further, where aninaccuracy or error follows a pattern, for example indicative of poorlyoptimized calibration constants used to correlate delay times with touchpositions, an error signal may be provided which ultimately prompts auser to remedy the problem, e.g. re-run calibration software.

Wave Modes

As noted earlier, this document defines "surface acoustic waves"(hereinafter "SAW") as acoustic waves for which a touch on the surfaceleads to a measurable attenuation of acoustic energy. Surface acousticwaves are used for the segments of acoustic paths traversing the desiredtouch sensitive zone of sensor subsystems. Other segments of theacoustic paths, e.g. along the arrays to and from the transducers, canuse any acoustic modes that couple to transducers and also couple to thedesired surface acoustic wave via scattering by the reflective array.Several examples of surface acoustic waves are known.

There are many advantages to Rayleigh waves including high touchsensitivity and high power density at the touch surface even forarbitrarily thick substrates. Rayleigh waves maintain a useful powerdensity at the touch surface due to boundary conditions that allows thesubstrate material to deform into free space, effectively reducingmaterial stiffness for the wave and hence its velocity, thus resultingin a reduced-velocity wave guiding effect near the surface. For example,this enables Rayleigh waves to be used on a CRT faceplate whichrepresents a very thick substrate relative to the acoustic wavelengths.

Like Rayleigh waves, Love waves are "surface-bound waves". Particlemotion is vertical and longitudinal for Rayleigh waves; both shear andpressure/tension stresses are associated with Rayleigh waves. Incontrast, for Love waves, particle motion is horizontal, i.e. parallelto the substrate surface. Only shear stress is associated with a Lovewave. Love waves have the advantage that they do not couple to water orother liquid or quasi-liquid contaminants, e.g. a silicone rubber seal,via pressure-wave radiation damping. Of course, elimination of radiationdamping will also reduce sensitivity for a finger touch which must nowrely on viscous damping alone. However, for market niches for whichliquid contamination is a particular problem, a Love wave may bepreferable to a Rayleigh wave. Depending on substrate design, theconcentration of acoustic power at the surface, and hence the touchsensitivity, can be varied. A key advantage of Love waves, or moregenerally asymmetric horizontally polarized shear waves, is that theymay have no appreciable energy on the lower surface of the substrate.

Another class of surface acoustic waves of possible interest inconnection with acoustic touchscreens are plate waves. Unlikesurface-bound waves, plate waves require the confining effects of boththe top and bottom surfaces of the substrate to maintain a useful powerdensity at the touch surface. Examples of plate waves include symmetricand anti-symmetric Lamb waves, zeroth order horizontally polarized shear(ZOHPS) waves, and higher order horizontally polarized shear (HOHPS)waves.

Use of plate waves constrains the thickness of the substrate. For platewaves, the touch sensitivity decreases with increasing plate thickness.For low-order plate waves such as ZOHPS waves, phase velocitydifferences with respect to neighboring acoustic modes (i.e. n=1 HOHPSfor ZOHPS) shrink with increasing substrate thickness making cleanseparation of modes more difficult. ZOHPS sensors operating at 5 MHz arethus typically limited to substrate thickness of about one millimeterfor glass. Higher order modes can be used with somewhat thicker glasssubstrates of 2 to 3 millimeters, in part because of a largerphase-velocity differences from neighboring modes, and in part becausenodal planes inside the substrate help concentrate acoustic power at thetouch surface. Note that plate waves are equally sensitive on the topand bottom of the substrate. For applications in which limitations onsubstrate thickness are not a problem, and for which bottom-side touchsensitivity is not a problem, plate waves are viable options.

The choice of acoustic mode effects touch sensitivity, the relativetouch sensitivity between water drops and finger touches, as well as anumber of sensor design details. However, the basic principles ofacoustic touchscreen operation are largely independent of the choice ofacoustic mode.

As noted above, there can be particular advantages to simultaneousdetection of touches with more than one acoustic mode. For example, awater rejection can be based on a comparison of touch perturbations froma horizontally polarized shear mode (e.g. Love, ZOHPS, HOHPS) and from amode that is damped by pressure-wave radiation in liquids (e.g.Rayleigh, Lamb).

Mode Distingushing Physical Filters

Sensor subsystems are designed for a particular set of acoustic pathsand acoustic modes. Ideally, the receiver signal is entirely due to thedesired paths and modes. With the aid of a phase sensitive controller,some interference from parasitic acoustic signals may be acceptable,nevertheless it is still desirable to avoid excessive parasiticinterference. Various physical filters may be employed to assure thatsufficient suppression of parasitic modes is achieved.

The transducers and arrays provide the most basic filters to suppressundesired acoustic modes. For example, a Rayleigh-wave transducercomposed of a pressure-mode piezoelectric element and a wedge willcouple strongly to Rayleigh waves but very weakly to possible parasitichorizontally-polarized shear waves. The arrays themselves are typicallyvery selective mode filters. Referring to FIG. 11, an array's spacingvector is designed to efficiently scatter the desired mode in thedesired direction, fortuitous circumstances are required for the arrayto coherently scatter the wrong mode or the right mode in the wrongdirection. In addition to these two fundamental mode filteringmechanisms that are inherently part of the sensor design, further modefilters may be introduced.

In some cases, the mode selectivity of reflective arrays can be enhancedby means of the depth structure of the reflective arrays. For example,FIGS. 5A and 5B of U.S. Pat. No. 5,072,427 consider reflective materialson both the top and bottom surfaces of a substrate in order to reduceparasitic mode generation during mode conversion from a ZOHPS wave to aLamb wave of a specified symmetry (symmetric or anti-symmetric). Inprinciple, optimal reflective array mode selectivity could be obtainedif one had full design control over the depth profile of the reflectorstructure; this is not always cost-effective in practice.

Commercial acoustic touchscreens typically use acoustic dampingmaterials to absorb or "filter out" undesired acoustic paths. Forexample, in sensor designs for which the substrate edges serve nodesired acoustic purpose, acoustic absorbers such as contact adhesivesare placed around at least part of the perimeter of the substrate toeliminate possible reflections off the edge of the glass. Such absorbersat the substrate perimeter are analogous in function to thewave-absorbing design of the sides of Olympic-style swimming pools;waves impinging the perimeter are not reflected back into the activearea where they may complicate signal analysis or slow down swimmers.Such prior art techniques may also be applied to embodiments of thepresent invention.

FIG. 14, reference numeral 52, of U.S. Pat. No. 5,177,327 also shows useof acoustically absorbing materials to differentially absorb desired andparasitic modes. Such techniques may also be applicable to embodimentsof the current invention. Reverse reflection schemes, such at that shownin FIG. 17 of U.S. Pat. No. 5,177,327, increase opportunities to placemode filters in the acoustic path from the transmit to receivetransducers.

The substrate itself may be designed to selectively propagate desiredmodes and cut-off or absorb undesired modes. For example, if the desiredmode is a lowest order plate wave, e.g. a ZOHPS or a flexural wave, thena sufficiently thin substrate will not support Rayleigh waves nor thehigher order plate waves for which the cut-off frequency is above theoperating frequency; the thinning may be in a limited region or for theentire substrate. As another example, the glass-polymer-glass laminationof safety glass may serve as a Rayleigh-wave substrate for wavelengthssufficiently small compared to upper glass thickness; all plate wavemodes will be absorbed by the acoustically absorbing adhesive bondinglayer. As yet another example, a Love-wave substrate may be designedwith a thin slow-velocity layer on the top, a higher velocity medium inthe middle, and an acoustically absorbing material on the bottom. Thethickness of the slow-velocity layer can be reduced until only thelowest-order Love wave is captured. Such a Love-wave substrate willsupport the lowest-order Love wave and a modified Rayleigh wave, butwill not propagate any of the (modified) plate-wave modes.

Some embodiments involve reflective boundaries. In some cases, thereflective boundary may be a source of parasitic mode conversions, forexample when the reflective boundary is simply a machined edge of thesubstrate. In other cases, the reflective boundary may provide furthermode filtering, for example reflective boundaries based on coherentscattering off of multiple reflectors, i.e. reflective boundaries basedon the principles of FIGS. 11 and 17.

One interesting variant of a reflective boundary is a beveled edge of asubstrate. As the substrate thins, cut-off frequencies for higher orderplate-wave modes increase. If the beveled edge becomes sufficiently thinto drive a cut-off frequency above the operating frequency, then thecorresponding mode will be reflected. Due to the high acoustic powerdensity at the reflection point (analogous to the long dwell time at themaximum elevation of an object rolling up a hill), an absorber placed atthe reflection point will have increased damping effect. For a beveledsubstrate edge, different higher order modes will have reflection pointsat different distances from the substrate edge; thus allowingstrategically placed absorbers to differentially absorb different modes.This basic mode-filtering technical applies to both Lamb waves andhorizontally polarized shear waves.

As an example, let us look at horizontally polarized shear waves in moredetail. A mode selective filter may be provided by a substrate having abeveled reflective edge. As the substrate becomes thinner, the cutofffrequencies for HOHPS modes increase. As the wave-guide cut offfrequency increases, the group velocity decreases. When the substrate isthin enough for the cut-off frequency to equal the operating frequency,the group velocity becomes zero and the wave is reflected. Thisreflection point differs for differing wave modes, such that the largerthe order n of the mode, the further from the edge of the substrate willbe the reflection point. An acoustic wave generally has enhancedsensitivity to absorbing materials placed in the neighborhood of thereflection point. The substrate beveling may take the form of anarrowing bevel or tapering on one or both sides of the substrate.

Thus, if an HOHPS of order n is desired, e.g., order n=4, the order n-1wave, e.g., order n=3, will be the wave with the closest group velocityand hence a potential source of interference. On a substrate with abeveled edge, an absorptive material, such as a sealant, RTV siliconeepoxy, adhesive or self-adhesive tape, placed beyond the order n=4reflection point (where the substrate is thinner) will filter the lowerorder waves. This material may be placed on one or both sides of thesubstrate. In order to filter the order n+1 wave, a further adsorbentmaterial may be placed at the order n+1 reflection point (where thesubstrate is thicker), which will absorb the order n+1 wave more thanthe order n wave due to the concentration of n+1 wave energy at thatpoint. For the special case than n=0, note that the n=1 HOHPS mode to besuppressed relative the ZOHPS mode. One attractive feature of such useof a beveled edge as a reflective mode filter is that glass and othersubstrates are commonly manufactured with beveled edges.

Positive Response Sensor

The scope of the present invention includes embodiments where one ormore sensor subsystems are of the positive-signal type. Here"positive-signal" refers to the use of desired acoustic paths for whicha touch induces a mode conversion required to complete the acousticpath, or produces a wave shifted in phase from the originating wave.Hence the signal perturbation is the generation of a signal amplitude ata delay time for which the previous signal amplitude was small or zero.Corresponding changes, if necessary, are made in the lower levelalgorithm. Embodiments using phase sensitive controllers may not requireany modifications; there is still a vector displacement in I-Q space.Once a delay time and perturbation magnitude are reconstructed, thehigher levels of the touch reconstruction algorithm proceed as theywould without the positive signal feature.

Before considering an illustrative example of a sensor usingpositive-signal sensor subsystems, recall the following acoustics.Consider the lowest order symmetric and anti-symmetric Lamb waveslabeled as L₁ and F₁ in, e.g., FIG. 10.18 of the second edition of"Acoustic Fields and Waves in Solids: Volume II" by B. A. Auld. In thelimit that the substrate is very thick, the phase velocities of thesetwo Lamb modes becomes the Rayleigh-wave velocity, and an in-phasesuperposition of L₁ and F₁, "L₁ +F₁ ", becomes a Rayleigh wave on thetop surface and an out-or-phase superposition of L₁ and F₁, "L₁ -F₁ ".If the substrate is thick, but finite in thickness, then thesesuperposed modes are "quasi-Rayleigh waves", which indeed are the"Rayleigh waves" used in actual touchscreens. For substrates of finitethickness, an acoustic wave initially launched as "L₁ +F₁ "quasi-Rayleigh wave on the top surface will convert into "L₁ -F₁ "quasi-Rayleigh wave on the bottom surface after propagating a distancegiven by

    d=(1/2f)v.sub.R.sup.2 /Δv

where f is the operating frequency, v_(R) is the Rayleigh-wave velocity,and Δv is the difference in phase velocity between the L₁ and F₁ modesat the operating frequency. By varying the thickness of the substrate,and can control the value of Δv and hence control the value of d. Forcommercial Rayleigh-wave sensors as are produced by Elo TouchSystems, itis desired that d be very large compared to the dimensions of thetouchscreen so that "Rayleigh waves" stay on the top surface of thetouch substrate. Use of 2 or 3 millimeter soda-lime glass at 5.53 MHzsatisfies this condition. Alternately U.S. Pat. No. 5,072,427 and5,162,618 teach acoustic sensors in which first order Lamb waves sensetouches; here the substrate is approximately two wavelengths thick orthinner, e.g. about 1 millimeter or less of soda-lime glass at 5 MHz.Here d is very short so that quasi-Rayleigh wave behavior is suppressed.Having reviewed this background material, let us know consider the caseof an intermediate substrate thickness (perhaps determined empirically)for which d is equal to the distance of an acoustic path across thetouch sensitive zone.

For simplicity, let us first consider the case in which there are noreflective arrays, but rather a straight line of sight betwen transmitand receive transducers as in U.S. Pat. No. 3,673,327 of Johnson andFryberger, as shown in FIG., 1(a). As described above, the substratethickness is chosen such that a top-surface quasi-Rayleigh wave launchedon one side of the touch region will be received as a bottom-surfacequasi-Rayleigh wave on the other side of the touch region. By placing,e.g. wedge, transmit and receive transducers on the top and bottomsurfaces of the substrate, both top and bottom quasi-Rayleigh waves canbe launched and received. This arrangement supports four sensorsubsystems with top-to-top, top-to-bottom, bottom-to-top, andbottom-to-bottom, acoustic transmission from a transmit transducer to areceive transducer. Before a touch (or contaminants), the top-to-bottomand bottom-to-top subsystems have large signals and the top-to-top andbottom-to-bottom subsystems have small (nominally no) signals. A touchwill attenuate the top-to-bottom and bottom-to-top signals and create apositive signal response in the top-to-top and bottom-to-bottom signals.

For this example, a strongly absorbing touch on the top surface of thetouch region will "reset" the wave mode as a bottom quasi-Rayleigh wave.Hence a touch not only attenuates the acoustic energy, but also changesthe phase between the L₁ and F₁ components. Hence the L₁ and F₁components will no longer have a phase difference of 0° or 180° at thereceive transducers, and hence the received wave will no longer be apure quasi-Rayleigh wave on one surface. The receive transducers on bothsurfaces will both detect a quasi-Rayleigh wave component. By thismechanism, the absorbing touch leads to a positive signal.

Note that a quasi-Rayleigh wave launched on the top surface andnominally received on the bottom surface will be more heavily absorbed(as measured by the sum of the intensities of the two receive signals)by a touch nearer the transmit transducers than the receive transducers.The converse is true for a quasi-Rayleigh wave launched on the bottomsurface. Hence the ratio of the summed received intensities for burstsfrom the two transmit transducers provides a measurement of positionalong the direction of the acoustic path. With the four signals,top-to-top, top-to-bottom, bottom-to-top, and bottom-to-bottom, a touchcoordinate and touch pressure can be determined with two additionaldegrees of freedom for redundancy or measurement of additional touchcharacteristics.

The above transducer-line-of-sight scheme generalizes to sensors withreflective arrays. For example, a rectangular sensor with only "X"arrays could measure X and Y coordinates if it is provided withtransducers and reflective arrays on both the top and bottom of thesubstrate. Note that in this scheme there is no ambiguity for multipletouches regarding which X coordinate to associate with which Ycoordinate. It may be advantageous to thicken the substrate in theregion of the arrays so that quasi-Rayleigh waves between the transducerand the scattering on the arrays stay on the desired surfaces. For sucha rectangular sensor with both "X" and "Y" arrays and transducers on thetop and bottom surface, both X and Y coordinates would be redundantlymeasured.

Positive-signal sensor subsystems may be considered in which a touchinduces other types of mode conversions, e.g. where a wedge transducertransmits a Rayleigh wave, the transmitting array scatters the acousticbeam into the touch region in the form of an n=3 HOHPS wave, thereceiving array selectively scatters n=4 HOHPS waves into Rayleigh waveswhich are then received by a Rayleigh-wave wedge transducer. Modeconversion from n=3 to n=4 HOHPS mode in the touch region is required tocomplete the acoustic path. Such a sensor would be sensitive to suchmode converting perturbations.

In an embodiment such a positive-signal sensor subsystem is combinedwith other sensor subsystems, and dual or multiple mode algorithmsprovide added characterizations of true and false touches.

Non-touchscreen Applications

The present invention adds much flexibility regarding in sensorgeometry. Maximum sensor size is increased. Sensors need not berectangular. Dramatically non-planar shapes are possible. This opens thedoor for many non-touchscreen applications for which rectangularsubstrates are not appropriate.

The hexagonal sensor of FIG. 15(b), the triangular sensor of FIG. 15(c),the cylindrical sensor of FIG. 19(a), the spherical sensors of FIG. 21,the basin sensor of FIG. 22(a) and (b), and the half-hemisphere of FIG.22(c) provide examples of the freedom of sensor geometry provided by theprinciples of FIG. 10, FIG. 17, and FIG. 20. This enhances theapplicability of acoustic touch sensor technology to, for example, endowrobot components with a sense of touch.

An example of a non-touchscreen application of acoustic touch positionsensing is to detect the presence of acoustically absorptive matter on asurface. For example, as shown in FIGS. 22(a) and (b), acoustic wavesare used to determine whether a basin or toilet has been used, thenature of its contents, and during a flushing or draining processwhether and when it is reasonably clean. Thus, a feedback mechanism isprovided for the inside of a wash basin or toilet, which may form partof a control system. If it is desirable that the sensor may be able todistinguish solids from liquids, it is preferred that horizontal shearwave such as a Love wave be employed. Such a wave is viscositysensitive, and thus is relatively insensitive to water and moresensitive to viscous materials. Algorithms may thus be implemented tosupport public policy, e.g., a recent California drought slogan: "Ifit's pee, let it be, if it's brown, flush it down." In this case, theposition of a mass need not be determined with precision or withoutambiguity.

The present invention allows the use of redundant information todetermine the position or quality of a touch, allowing differentialsensing and immunity from shadowing in various embodiments. While oftenit is desired to sense coordinate position, in some applications, anaxial position measurement is sufficient. Therefore, the presentinvention also includes one dimensional sensors. For example, practicalapplications of 1-D sensors include components of an apartment buildingdoor-bell system; elevator buttons; musical devices; single degree offreedom manual input devices; and a touch sensor for a security entrysystem. For example, See FIG. 27, which shows a dual mode primarilysingle axis sensing system. In this system, waves of differing modes, aRayleigh wave and an n=4 HOHPS wave are employed. Both waves are emittedfrom a single transducer 2701. The wave is emitted at about 5 MHz, isscattered at a non-orthogonal angle by a reflective array 2702, and isscattered to a second transducer 2703 by another reflective array 2704.The HOHPS wave, on the other hand, is emitted and travels along anorthogonal path 2705. The HOHPS wave passes through the reflective array2704, reflects of a selective n=4 HOHPS reflective boundary 2706, formedas a partially masked beveled edge, and is redirected along its path ofincidence, to the reflective array 2702, back to the transducer 2701. Inthis case, the substrate 2707 is aluminum. FIG. 27(b) shows a timingdiagram for the system according to FIG. 27(a), in which the Rayleigh,FIG. 27(b)(1) and HOHPS, FIG. 27(b)(2), waves overlap in time. Due tothe non-orthogonal path of the Rayleigh wave, some horizontal positioninformation may be extracted from the received signals.

For security systems, for example, it may be advantageous to be able toverify that the finger or stylus has the expected acoustic properties(e.g., ratio of shear to Rayleigh absorption). Thus, for example, toenter an area containing biohazard materials, one may want to requirethat the operator is wearing dry rubber gloves selected, in part, tohave a distinctive acoustic signature.

The use of coversheets extend the potential applications for thisinvention. The touch surface may be exposed, or covered. For example, aplastic sheet with spacer dots, provides a number of interestingadvantages. Such a cover sheet protects touch surfaces from scratchesand digs that may disrupt propagation of acoustic waves; particularly ifthe shell is a soft metal like aluminum. By keeping rain off the touchsurfaces, a cover sheet may enable outdoor applications. When provided acoversheet, an acoustic sensor responds more directly to the pressure ofcontact via the compliance of the sheet (rather than sensing theacoustic properties of the contacting material). See, Knowles and Huang,U.S. Pat. No. 5,451,723, which proposes the use of a cover sheet withacoustic touch panels for certain applications.

Advantageously, "control buttons" can be painted on the touch surface,or on a corresponding cover sheet. This allows a single surface to serveboth as sensor and a user input device, providing an opportunity for aunified interface to reduce costs, such as on a toy device, or to bestexploit available surface area. On a toy robot, for example, acylindrical aluminum shell is provided with a cover sheet havinggraphics. As the toy robot moves, any contact will be sensed. When therobot is motionless, or under other circumstances where contact withenvironmental objects is unlikely, e.g., where a touch is not detectedalong an axis of movement or expected touch, the sensing may be analyzedas a potential input from a user.

The scope of this invention is not limited to transparent touchscreensplaced in front of display devices.

EXAMPLES Example 1

FIG. 10 represents a generic sensor subsystem including transmit andreceive transducers and arrays. This generic sensor subsystem includes afamily of embodiments which differ in the various parameters.

Later examples will generalize to families of embodiments in which theacoustic path between the transmit and receive arrays are scattered oneor more times by reflective boundaries. Later examples will alsogeneralize to families of embodiments in which the sensor surface hasnon-planar geometry. In this example, we consider the generic sensorsubsystem of FIG. 10.

In the embodiment of FIG. 10, a set of acoustic paths is associated witha sensor subsystem. Each member of the set of acoustic paths can berepresented by a value between zero and one of a path parameter "s". (Incases where a range other than zero to one is mathematically convenient;this can easily be accommodated with a change of variables.) Theacoustic path for a given path parameter starts at the transmittransducer centered at position T_(t), proceeds along the transmit arrayfor a distance and direction sA_(t) where it is redirected across thetouch sensitive region for a distance and direction P(s) where it isintercepted by the receive array. The acoustic path then continues for adistance and direction given by sA_(r) until it ends at the receivetransducer centered at T_(r). The array locations and orientations neednot have any particular relation to Cartesian axis directions or edgesof the glass.

According the present invention, there is no requirement that the set ofacoustic paths through the active region be a set of parallel paths,although in many cases it is convenient to do so in order to simplifytouch reconstruction algorithms.

There is an acoustic mode associated with each segment of the acousticpaths. Here we define V_(t) to the group velocity, and v_(t) to be thephase velocity, of the acoustic mode that is emitted from the transmittransducer and travels down the transmit array and is scattered, andperhaps mode converted, by the transmit array. Similarly we define V_(p)and v_(p) for the path segment across the touch region and V_(r) andv_(r) for the segment along the receive array. The acoustic mode acrossthe touch region must be a surface acoustic wave such as Rayleigh orLove waves and Lamb, ZOHPS, HOHPS waves in sufficiently thin substrates.The modes along the arrays can be any mode with sufficient coupling tothe transducers and the reflective array elements.

For each value of the path parameter, there is an associated delay time.This delay time is related to the group velocities of the modes used bythe following equation.

    t(s)=sA.sub.t /V.sub.t +P(s)/V.sub.p +sA.sub.r /V.sub.r

(Recall notation where, for example, A_(t) represents the magnitude ofthe "bold face" vector A_(t).)

It is generally desirable, but not a requirement, that a geometry bechosen so that the delay time is a monotonically increasing function ofthe path parameter. If so, then a finger touch in the active regioncausing a reduction in signal amplitude at time t(s) must be locatedwithin the locus of points defined by varying α between zero in one inthe following expression.

    αP(s)+sA.sub.t +T.sub.t

The reflective arrays may be designed with the aid of the spacing-vectormethod illustrated in FIG. 11. Wave vectors used in the spacing-vectorcalculations are determined as follows. The wave vectors k_(t), k_(p)(s), and k_(r), are parallel to A_(t), P(s), and A_(r) respectively. Themagnitudes of the wave vectors k_(t) =2π/λ_(t), k_(p) =2π/λ_(p), k_(r)2π/λ_(r), dependent on the mode wavelengths λ_(t), λ_(p), and λ_(r),which in turn are determined by the operating frequency, f, and thephase (not group) velocities v_(t), v_(p), and v_(r), namely λ_(t)=v_(t) /f, λ_(p) =v_(p) /f, and λ_(r) =v_(r) /f. Given the wave vectorsas a function of the path parameter, the reflective array spacingvectors may be calculated as follows. For the transmit array, thereflector spacing vector can be calculated by the equation below.

    S(s)=2πn(k.sub.t -k.sub.p (s))/|k.sub.t -k.sub.p (s)|.sup.2

Similarly, for the receive array, the orientation and spacing of thereflectors are determined as follows.

    S(s)=2πn(k.sub.p (s)-k.sub.r)/|k.sub.p (s)-k.sub.r |.sup.2

For clarity of presentation, FIG. 10 does not fully does not include thefollowing generalizations which are within the scope of this invention.

The vector representing the receive-array scatter location, sA_(r), ismore generally expressed as r(s)A_(r) where r(s) is a monotonicallyincreasing function of the path parameter with a range from zero to one.

FIG. 11 of U.S. Pat. No. 4,642,423 of Adler teaches an array designtechnique to cause an acoustic beam traveling along an array to deviatefrom a straight-line trajectory, or more generally to deviate from ageodesic trajectory such as a great circle. With the use of this orother wave guiding techniques, the family of embodiments associated withFIG. 10 is extended to cases in which the arrays are not straight linesegments.

A gap without reflectors can be introduced between the transducers andtheir corresponding arrays. Such a gap, for example, may be used toprevent the transducer in question from blocking acoustic paths ofanother sensor subsystem. More generally, within the formalism of FIG.10, there is no requirement that the full length of the array ispopulated with reflective elements.

Example 2

FIG. 12 illustrates examples of specific touch region geometries withinthe scope of FIG. 10.

The rectangular touch region shown in FIG. 12(a) is typical of theX-coordinate sensor subsystems of current flat rectangular acoustictouchscreen products.

If all segments of the acoustic path in FIG. 12(a) use the same acousticmode, then the spacing-vector formula leads to the prior art 45°reflectors with wavelength spacing along the array axes. For example, ifthe incident wave is in the -X direction and the scattered wave is inthe Y direction, then the spacing vector is calculated to beS=(nλ/2,nλ/2) and hence the reflectors are at 45° the spacing betweenreflectors in a direction perpendicular to the reflectors is nλ/√2 andthe reflector spacing in along the array axis nλ.

If, instead, the transmit and receive array modes are zeroth orderhorizontally polarized shear (ZOHPS) waves and the touch region mode isa Lamb wave, then the spacing vector is calculated to be S=n (λ_(Lamb),λ_(ZOHPS)) {λ_(Lamb) λ_(ZOHPS) /(λ_(Lamb) ² +λZOHPS²)}. This in turnimplies a reflector angle θ with respect to array axis satisfying thecondition tan(θ)=λ_(Lamb) /λ_(ZOHPS) =v_(Lamb) /v_(ZOHPS). Thisreflector orientation is that given in equation 2 of U.S. Pat. No.5,072,427. In terms of θ, the magnitude of the spacing vector can bereduced to S=n sin(θ) λ_(ZOHPS) which in turn implies nλ_(ZOHPS)reflector spacing with respect to the array axis. Hence, the arraydesign essentials of U.S. Pat. No. 5,072,427, incorporated herein byreference, are also derivable from the spacing vector formula.

Going beyond prior art, the principles of FIG. 10 combined with thevector-spacing formula also allow one to engineer sensor subsystems fora variety of other geometries including parallelograms as in FIG. 12(b),trapezoids as in FIG. 12(c), and triangles, as shown in FIGS. 12(d),12(e) and 12(f).

Example 3

FIG. 13 represents an example sensor design that utilizes four sensorsubsystems. This design supports control systems which includeanti-shadowing algorithms and algorithms that resolve multi-touchambiguities. The touch region has a width-to-height aspect ratio of√3:1.

Two sensor subsystems are rectangular as in FIG. 12(a), see FIG. 13(a).By itself, FIG. 13(a) has much in common with prior art sensors withstandard X and Y measurements. FIG. 13(a) shows an X transmit transducer1301, an X transmit reflective array 1302, with reflective elements at45°, an X receive reflective array 1303, with reflective elements at45°, and an X receive transducer 1304. Likewise, a Y transmit transducer1305, a Y transmit reflective array 1306, with reflective elements at45°, a Y receive reflective array 1307, with reflective elements at 45°,and a Y receive transducer 1308 is provided along an orthogonal set ofaxes.

The other two subsystems provide a measurement of a "U" diagonalcoordinate. FIG. 13(b) illustrates these other two sensor subsystems.They are both of the type illustrated in FIG. 12(f). One set of acousticpaths start at the X transmit transducer 1301 and are received by the Yreceive transducer 1308; the touch region for this sensor subsystem istriangular with sides defined by the X transmit array 1302, the Yreceive array 1307, and a diagonal across the touch region. The otherset of acoustic paths start at the Y transmit transducer 1305 and arereceived by the X receive transducer 1304; the touch region for thissensor subsystem is triangular with sides defined by the Y transmitarray 1306, the X receive array 1303, and a diagonal across the touchregion. In both cases, the acoustic paths through the touch region areat a 30° diagonal with respect to the X axis. Together, the touchregions of these two sensor subsystems cover substantially all of therectangular touch region covered by the X and Y sensor subsystems ofFIG. 13(a).

Reflector angles and spacings are noted in FIG. 13. These values can bederived from the principles of FIGS. 10 and the spacing-vector formula.Reflector angles have been calculated for the case in which all segmentsof all acoustic paths use the same acoustic mode a wavelength λ. Thespacing-vector formula supports generalization to embodiments involvingmode conversion.

The arrays shown in FIGS. 13(a) and 13(b) are superposed. The transmitarray 1302 in front of the X transmit transducer includes both 45° and75° degree reflectors. The transmit array 1306 in front of the Ytransmit transducer includes both 45° and 60° degree reflectors. Thereceive array 1303 in front of the X receive transducer includes both45° and 15° degree reflectors. The receive array 1307 in front of the Yreceive transducer includes both 45° and 30° degree reflectors. Thesereflector orientations are appropriate for designs in which no modeconversion at the reflective arrays, i.e. reflector elements areoriented like mirrors so that the angle of incidence equals the angle ofreflectance.

In a specific embodiment for which the acoustic wavelength is 0.0226",e.g. soda-lime-glass substrate with an operating frequency of 5.53 MHz,the reflector spacings along the array axes are an integer times 0.0226"for all 45° reflectors, an integer times 0.0121" for the 75° X_(t)reflectors, 0.1687" for the 15° X_(r) reflectors, 0.0151" for the 60°Y_(t) reflectors, and 0.0452" for the 30° Y_(r) reflectors.

FIG. 23 shows the timing of received signals for the all-Rayleigh X-Y-U30°-diagonal sensor shown in FIG. 13. Other options for acoustic modesand geometrical dimensions of an X-Y-U diagonal sensor will lead toqualitatively similar timing diagrams. As shown, a single acousticemission from either transmit transducer is received by two receivingtransducers. It is noted that the diagonal axis is read in two halves,from separate transducers, hence, a touch 2301 will perturb only onesuch diagonal path signal. Compare FIGS. 23(d) and 23(c).

In order to analyze the received signals, an anti-shadowing algorithmmay be applied which first searches for a touch in one axis and then theother, reporting a valid output if both are found. If one is found andthe other is not, the diagonal axis signals are analyzed for touchinformation. If one of the diagonal signals indicates a touch, then thatsignal is transformed to produce the missing data and output. Where atouch is not found on another axis, an error condition exists. Where notouch is seen on any axis, no output is reported. With such ananti-shadowing algorithm, consider the case in which an acoustic wavesensing a touch of one sensor subsystem is strongly shadowed by, forexample, a drop of water absorbing a Rayleigh wave or a frozen drop ofwater absorbing a ZOHPS wave. In order to determine a position of atouch, two coordinates are required, which correspond to an X and Y.Where one of the X and Y is subject to interference, the 30° diagonalpath which intersects the touch position, but generally does not alsointersect the interfering substance, may be analyzed to determine theposition of a touch. A relatively simple transform is applied to produceoutputs corresponding to X and Y.

Alternately, a redundancy-check/multiple-touch algorithm may beemployed. Because the data includes redundant information, in theabsence of interference, the additional data may be used to check thecomputed positions for consistency or to determine the existence andposition of multiple touches. In an initial step, candidate X and Ytouch coordinates are constructed from the X and Y data streams. Thesecandidates are compared for consistency with the diagonal data. Whereall three correspond, a valid output condition exists. Where they do notcorrespond, a different candidate coordinate is proposed and tested.

Example 4

The sensor of example 3 and FIG. 13 is an X-Y-U sensor in which the Uacoustic paths are at a 30° diagonal angle and an aspect ratio of √3:1.In this example, we consider an X-Y-U sensor with an arbitrary aspectratio.

Let H and W be the height and width of the touch region of an X-Y-Usensor. The U acoustic paths through the touch region have a diagonalangle satisfying tan(Θ)=H/W. The principles of FIG. 10 and thespacing-vector formalism lead to the reflector angles and spacings forthe U subsystems as given in the table below. For the value Θ=30°, theangles and spacings of FIG. 13 are reproduced. The line widths given inthe table correspond to the line width that maximizes the scatteringamplitude; as previously noted these line widths may be used but are notrequired.

    ______________________________________                                        Angle         Spacing     Line width                                          ______________________________________                                        X transmit                                                                            90° - Θ/2                                                                  (1 + cos(Θ))                                                                        (4 × cos(Θ/2))                      X receive                                                                             Θ/2 (1 - cos(Θ))                                                                        (4 × cos(90° - Θ/2))         Y transmit                                                                            45° + Θ/2                                                                  (1 + sin(Θ))                                                                        (4 × cos(45° - Θ/2))         Y receive                                                                             45° - Θ/2                                                                  (1 - sin(Θ))                                                                        (4 × cos(45° + Θ/2))         ______________________________________                                    

In prototypes constructed according to the present invention, a systemhaving touch region dimensions of H=6.40" and W=9.60" was constructed.These are the same touch region dimensions as the commercial touchscreenproduct E284A-693 sold by Elo TouchSystems, Inc. The prototypes weresimilar in construction, including placement of reflective arrays,except that the arrays in the prototype sensor were superposed arraysincluding U reflectors as well as standard X and Y reflectors. Theseprototypes demonstrated the use of diagonal acoustic paths andsuperposed arrays. The U diagonal angle of the prototypes is 33.7° asrequired by tan(Θ)=H/W=6.40"/9.60"=tan(33.7°). The spacings andorientations of the reflectors for the U sensor subsystems werecalculated using the formulas in the above table.

All desired sets of acoustic paths were observed in the prototypes. Thisdemonstrates the operation of superposed arrays. This also providesanother demonstration of the spacing-vector formalism as a general meansto enable calculation of reflector spacings and orientations from firstprinciples. Some signal artifacts from interfering parasitic acousticpaths were observed with these prototypes, but none that can not beremoved or tolerated with the aid of one or more of the followingparasitic suppression techniques: reduction of electronic cross-talk incontroller and in cabling between X and Y burst lines; fine tuning ofarray foot print geometry; acoustic damping on back side of substrate(e.g. optical bonding or back side application of an OCLI HEA™anti-reflective coating on a plastic film with contact adhesive); linewidth modulation; increased density of reflectors elements; and use of aphase-sensitive controller. In all cases in which parasitic acousticpaths were detected in the prototypes, the acoustic path from thetransmit to the receive transducers involved only one scattering off ofan array; no signal artifacts were observed due to parasitic pathsinvolving multiple reflections.

Example 5

FIG. 14 provides for the addition of a second diagonal "V" measurementbeyond the X, Y, U sensor of FIG. 13. FIG. 14 therefore illustrates twoadditional sensor subsystems. The reflective arrays for this V subsystemare superposed on X,Y, and U arrays similar to those FIG. 13. Acousticpaths for one sensor subsystem start at the X transmit transducer 1301and end at the Y transmit transducer 1305, which in this context servesas a receiving transducer; this sensor subsystem is of the type shown inFIG. 12(e). Acoustic paths for the other sensor subsystem start at the Yreceive transducer 1308, now used as a transmit transducer, and arereceived by the X receive transducer 1304 transducer; this sensorsubsystem is of the type shown in FIG. 12(d). In this case, the Xtransmit reflective array 1302 includes an additional set of reflectiveelements at 15°, the X receive reflective array 1303 includes anadditional set of reflective elements at 75°, the Y "transmit"reflective array 1306 includes an additional set of reflective elementsat 30°, and the Y "receive" reflective array 1307 includes an additionalset of reflective elements at 60°. The reflector spacings are noted onFIG. 14, and may be calculated according to the principles according toFIG. 10 and the spacing-vector formula.

There are many options regarding the acoustic modes which may beemployed. In one embodiment, the U and V subsystems involve no modeconversions, while the X and Y subsystems do. For example, all acousticpath segments may use Rayleigh waves, except for the touch regionsegments of X and Y which use Love waves. Alternatively, all acousticpath segments may use ZOHPS waves, except for the touch regions of X andY which use a lowest order symmetric or anti-symmetric Lamb wave. Inboth these cases, one of the pairs of coordinates (X,Y) and (U,V) sensestouch with a horizontally polarized shear wave and the other with anacoustic mode with significant vertical motion (coupling to pressurewaves in fluids) at the touch surface. A false touch due to a water dropmay be rejected by an anomalously small ratio of shear-wave tonon-shear-wave touch signal.

If there is no mode conversion in the U and V subsystems, the principlesof FIG. 10 and the spacing-vector formula lead to the reflector anglesand spacings shown in FIGS. 13(b) and 14. However, where modeconversions are implemented, different reflector angles are used. Toimplement mode conversion in the X and Y subsystems, the 45° reflectorangles illustrated in FIG. 13(a) are not used; the tangent of thesereflector angles will equal the ratio of the phase velocity of the modein the touch region to the phase velocity of the mode propagating alongthe array.

As with the X-Y-U sensor, the X-Y-U-V sensor need not be limited todiagonal paths at +3° and -30° angles. The U and V angles areapproximately equal and opposite with magnitudes equal to the inversetangent of the aspect ratio of the touch region. Preferably, thediagonal waves have a angle of between about 10° to 80° to thereflective array, and more preferably between 25° and 65° to thereflective array. The various wave angles preferably differ from eachother by about at least 5°, and more preferably by at least 15°.

It should be understood that, as an extension of these techniques, it ispossible to direct a set of acoustic paths having an incrementalvariation from each transducer to all transducers, including theoriginating transducer. Thus, the system is not limited to, for example,four sets of acoustic paths. Likewise, while separating the sets ofreceived waves at differing transducers allows use of simple AMreceivers for the acoustic waves, the invention is not so limited and aplurality of sets of waves may be received by a single transducer, withportions received simultaneously.

Example 6

Touch sensors according to the present invention need not berectangular. A wide variety of polygonal shapes are possible. Forexample, FIG. 15(b) illustrates a hexagonal sensor including sixidentical trapezoidal sensor subsystems of the type of FIG. 12(c).

In this figure, even numbered transducers 1502, 1504, 1506, 1508, 1510,1512 are transmit transducers and odd numbered transducers 1501, 1503,1505, 1507, 1509, 1511 are receive transducers, although this is not theonly possibility. The six trapezoidal sensor subsystems provide acousticpaths between pairs of transducers as follows: transmit from 1512 withreceive at 1509; transmit 1502 with receive at 1511; transmit 1504 withreceive at 1501; transmit 1506 with receive at 1503; transmit 1508 withreceive 1505; and transmit 1510 with receive 1507.

The reflector angles of ±60° shown in FIG. 15(a) correspond to the casein which there is no mode conversion during scattering at the arrays.This can easily be generalized to other acoustic-mode options using thevector-spacing formula.

Most of the touch panel surface is covered by three sensor subsystems.If, as drawn, arrays are shortened to make room for transducers, thenthere may be the three strips covered by only two sensor subsystems. Inthis case, the very center of the panel is insensitive. Alternately, onecan back off the transducers and use longer arrays; perhaps at theexpense of increasing the width of the border region. In addition, a setof paths may be provided which are rectangular, e.g., FIG. 12(a), whichencompass the gap region. Three such added rectangular sensor subsystemsmay utilize the following pairs of transmit/receive transducers: 1506and 1501; 1508 and 1503; and 1510 and 1505. When these three rectangularsensor subsystems are added, then each array has three superposed setsof reflective arrays.

In theory, a reflective array system may be provided to direct acousticwaves to each other reflective array (with a reflection allowingtransmission to reflective arrays disposed on the same side or at anoverly obtuse angle). It is preferred, however, in order to minimizeacoustic losses, that no more than three sets of reflective elements beprovided in any reflective array.

This hexagonal touch sensitive surface has application, for example, asa system built into a conference table. The sensor is provided as, forexample, a portion of a table top for a round table. A central portionof the table includes a hexagonal substrate, each edge having a pair oftransducers and a composite reflective array having two or three sets ofreflective elements for directing waves to each side which is notadjacent. Thus, with three superposed arrays, waves are directed at 60°,90° and 120°. Therefore, to sense coordinate position along two axes,potentially 18 wave paths are provided, or nine non-trivial paths. Asignificant redundancy therefore exists, providing enhanced immunity toshadowing and allowing resolution of multiple simultaneous touches,suppression of interference from fixed objects, distinction of fingerfrom palm, and other advantages. Several people sitting around the tablecould simultaneously interact with the system. Due to the large numberof sensing paths, this table therefore allows conferencing andmultitouch capability. The touch panel itself may be opaque ortransparent. If it is transparent, a reverse projection screen may beprovided on the back side of the substrate upon which an image isprojected. For example, a projective display device may illuminate areverse projection screen laminated to the underside of a 3 mm thickborosilicate glass substrate, upon which an all-Rayleigh wave hexagonalsensor is fabricated; the relatively low acoustic attenuation ofborosilicate glass at 5.53 MHz can support a sensor size with a touchregion large enough to contain a 60 cm diameter circle.

As shown in FIG. 15(c), the principles may also be applied to atriangular sensor, which in this case also uses the arrays according toFIG. 15(a). In this case, the wavepaths correspond to FIG. 12(d).Alternatively, with array designs differing from FIG. 15(a), atriangular sensor may use three sensor subsystems of the type of FIG.12(e) with the same arrangement of transmit and receive transducersgiven in FIG. 15(c).

As is evident from this example, and the general principles of FIGS. 10and 11, a wide variety of polygons are possible shapes for touchsensors.

Example 7

This example illustrates the use of this invention to extend sensorsize. For some applications, e.g. electronic whiteboards, larger sensorsizes are of considerable interest.

For a given acoustic mode, operating frequency, and acoustic substrate,the sensor size is limited by a maximum acoustic path length beyondwhich further attenuation of the acoustic waves leads to unacceptablyweak signal strengths. For example, for 5.53 MHz Rayleigh waves insoda-lime glass, weak signal amplitudes typically are a problem foracoustic paths longer than about one meter. Use of borosilicate glass oraluminum as a substrate can almost double this maximum path length, forotherwise the same system parameters.

The characteristics of the receiver systems and electronics, theelectromagnetic noise environment affect the limit of detectability andhence affect the numerical value of the maximum acoustic path length.The efficiency of transducers and reflective arrays also influence themaximum path length. Whatever factors limit the acoustic path length,for a given maximum path this invention provides means to extend themaximum size of the sensor. For example, as shown in FIG. 16, a largerectangular sensor is shown having an X, U, V sensor layout along withrepresentative examples of sensor subsystems.

For the X sensor subsystem of a prior art rectangular touchscreen, themaximum path length is approximately 2W+H where W and H are the widthand height of the touch region. In the sensor illustrated in FIG. 16,the width is divided into four segments of length S=W/4. Hence each ofthe X sensor subsystems has a maximum acoustic path length ofapproximately 2S+H=W/2+H. For the U and V sensor subsystems, the maximumpath length is 3S/2+√[H² +(3S/2)² ]=3W/8+√[H² +(3W/8)² ]. For a touchregion with a 3-to-4 aspect ratio, known prior art rectangulartouchscreens employ a maximum path length of 2W+H=(11/3)H, while FIG. 16gives maximum path lengths of W/2+H=(5/3)H and 0.46W+H=1.62H<(5/3)H. Forgiven sensor size. the maximum acoustic path has reduced by more than afactor of two. For a given maximum acoustic path length the sensor sizecan more than doubled. By avoiding long acoustic path lengths alongarrays, the permissible path lengths crossing the touch region areincreased. This increases the maximum permissible height of therectangular sensor.

According to this embodiment, as shown in FIG. 16, acoustic paths thatmust traverse the entire width of the sensor, e.g. as found in a Ysensor subsystems, have been avoided. As a result, there is noacoustic-path-length limitation on the width of the large rectangularsensor, provided there is no limitation on the number of transducers.Thus, while the height of the sensor in FIG. 16 is extended by remainslimited, the width of the sensor can be arbitrarily increased withfurther segmentation of the arrays.

Delving into further details of the sensor in FIG. 16, there are sixtransmit transducers as indicated at the top of the sensor labeled TL,T1, T2, T3, T4, and TR. Six receive transducers are indicated at thebottom of the sensor and are labeled RL, R1, R2, R3, R4, and RR. Thearrays associated with T1, T2, T3, T4, R1, R2, R3, and R4, are triplysuperposed arrays containing reflectors for X, U, and V sensorsubsystems. The vertical arrays are doubly superposed arrays supportingU and V sensor subsystems only.

The reflector angles and spacings can be computed from the principles ofFIGS. 10 and 11. The acoustic paths across the touch region are atangles of 90° and 90°±θ with respect to the horizontal where θ isdefined as arctan[(3/2)S/H]. If there is no mode conversion, thereflective arrays at the top and bottom of the sensor have reflectors atangles of 45° and 45°±θ/2 and reflector spacings of nλ and nλ(1±sin θ).The side arrays have reflector angles and spacings of ±θ/2 and nλ(1-cosθ). One option for the reflector width can be determined by equalizingthe reflector line widths and the gaps between reflectors for thedensest reflector spacing. The spacing-vector formula allows calculationof reflector design parameters for embodiments employing modeconversion.

To minimize dead regions, it may be advantageous to "shingle" the topand bottom transducer-array systems, as shown in FIG. 16(c). By tiltingthe arrays so that the transducer end of the arrays is at least onetransducer width closer to the edge of the glass than the far end of thearrays, the arrays can be extended to eliminate the dead spaces causedby the transducers as shown in FIG. 16(b).

The X, U, and V, coordinates determined by the sensor subsystems arelinearly related to the delay times for signal perturbations induced bytouches. Each sensor subsystem has is own appropriate constants for thislinear mapping. U and V are related to Cartesian coordinates by thefollowing relations.

    U=X+tan θ×Y V=X-tan θ×Y

The X, U, and V coordinates are interrelated by the following equations.

    X=(U+V)/2 U=2X-V V=2X-U

Hence the Cartesian coordinates (X, Y) are related to X, U, and V asfollows.

    (X,Y)=({U+V}/2,{U-V}/{2×tan θ})=(X,{U-X}/tan θ)=(X,-{V-X}/tan θ)

Any two of X, U, and V is sufficient to determine a touch position.Furthermore, due to the redundancy and segmentation of the touchsensitive surface, multiple touches may be simultaneously detected andtheir positions analyzed.

Now consider specific embodiments with no mode conversion having arectangular touch region with a 3-to-4 aspect ratio. The value for θ is26.6°, so that the waves traverse the touch region at angles of 64.4°,90° and 16.6° with respect to the horizontal X direction. The Usubsystem reflectors angles are 31.72° for the top arrays, 58.28° forthe bottom arrays, and 13.3° for the side arrays. The X subsystemreflectors of the top and bottom arrays are at 45°. The V subsystemreflector angles are 58.28° for the top arrays, 31.72° for the bottomarrays, and 13.3° for the side arrays. The U subsystem reflectorsspacing is 1.809×nλ for the top arrays, 0.691×nλ for the bottom arrays,and 9.472×nλ for the side arrays. The X subsystem reflectors of the topand bottom arrays have nλ spacing. The V subsystem reflector spacingsare 0.691×nλ for the top arrays, 1.809×nλ for the bottom arrays, and9.472×nλ for the side arrays.

Further, another embodiment of the sensor has a layout wherein thedimensions of the touch region are 3 foot by 4 foot, i.e. 60 inchdiagonal. With the layout of FIG. 16(a), the maximum acoustic pathlength is 60". This is feasible for 5.53 MHz Rayleigh waves in aborosilicate or aluminum substrate. Hence, the sensor design of FIG.16(a) supports a size typical of many electronic white boards presentlyon the market.

White boards (not necessarily electronic) often have an enamel-on-metalwriting surface. An aspect of the present invention is the use of aLove-wave substrate composed of a thin layer of low-acoustic-velocityenamel on a low-acoustic loss metal substrate. For example, anapproximately 100 μm thick lead (or other heavy metal) based enamel onan 3 mm aluminum substrate, similar to known architectural panels inpast use. Such a Love-wave substrate may be advantageous in suppressingeffects of liquid contaminants such as a trail of drying ink from a feltpen and furthermore may be able to provide greater touch sensitivitythan ZOHPS waves in a one millimeter thick substrate due to the Lovewave's higher concentration of acoustic power near the surface.

Example 8

Flexibility of sensor design is further enhanced by the use ofreflective boundaries in sensor subsystems to produce intermediatescattering of acoustic paths. See FIG. 17 which is a generalization ofFIG. 10 in which the acoustic paths undergo an intermediate reflectionbetween the transmit and receive arrays. Similarly, a plurality ofreflections may occur.

The reflective boundary 1701 may be formed of an array of reflectiveelements similar to the transmit and receive arrays. For example seeitem 60 of FIG. 11 of U.S. Pat. No. 4,700,176. The angles and spacing ofreflectors may be calculated using the spacing-vector formula, that isthe reflective boundary's reflector spacing vector is given by thefollowing expression (subscript "pt" for path from transmit array and"pr" for path to receive array).

    S(s)=2πn(k.sub.pt (s)-k.sub.pr (s))/|k.sub.pt (s)-k.sub.pr (s)|.sup.2

As with transmit and receive arrays, mode conversion is an option at areflective boundary 1701.

Unlike the transmit and receive arrays, which need to bequasi-transparent to acoustic paths propagating along their lengths,reflective boundaries 1701 can be designed to be strongly reflective, solong as no acoustic path need pass through the reflective boundary 1701.

As with transmit and receive arrays, reflective boundary 1701 arraysfrom different sensor subsystems can also be can superposed with anothersensor subsystem's reflective boundary 1701 or reflective array.

The reflective boundary 1701 may include an edge of the substrate. Inthis case additional diffraction-grating reflective boundary schemes maybe considered, such as a faceted edge. If no mode conversion is desired,and the angle of incidence equals the angle of reflectance, then asimple machined glass edge may be sufficient, particularly for thelowest order plate waves: ZOHPS and lowest order symmetric andanti-symmetric Lamb waves. For example, see items 220 and 222 of FIG. 17of U.S. Pat. No. 5,243,148.

While the acoustic path in FIG. 17 encounters only one reflectiveboundary 1701, these principles generalize to two or more intermediatereflections.

The delay time in terms of path parameter and group velocities of modesalong transmit array (V_(t)), transmit path across touch region(V_(pt)), transmit path across touch region (V_(pr)), and receive array(V_(r)) is

    t(s)=sA.sub.t /V.sub.t +P.sub.t (s)/V.sub.pt +P.sub.r (s)/V.sub.pr +sA.sub.r /V.sub.r

A perturbation in the signal at this time corresponds to a touch withinthe union of the following two sets of points (where 0<α<1).

    αP.sub.t (s)+sA.sub.t +T.sub.t

    αP.sub.r (s)+P.sub.t (s)+sA.sub.t +T.sub.t

FIG. 11 of U.S. Pat. No. 4,700,176 illustrates the use of reflectiveboundaries to reduce the required number of transducers; a sensorsubsystem is shown in which the acoustic path leaves a transducer, isscattered by 90° by the reflective array, traverses the touch region, isreflected back across the touch region by a reflective boundary, andretraces its path back to the same transducer from which it started.

FIG. 17 of U.S. Pat. No. 5,243,148 illustrates the use of reflectorboundaries to pass the acoustic path through mode selective filters. Inthis example, the "reverse reflection" reflective array directs acousticpaths away from the touch region through a mode filter before areflective boundary redirects the acoustic paths back toward the touchregion.

In the context of the present invention, reflective boundaries furtherincrease the options for non-orthogonal acoustic paths, the optionsnon-rectangular and non-planar sensor shapes, and the options forsensing touches with more than one acoustic mode.

Example 9

FIG. 17 embodies many possible sensor subsystem geometries, a fewexamples of which are show in FIG. 18.

In FIG. 18(a), the receive and transmit transducers and arrays are onone side of a trapezoidal touch sensitive region. The receiver andtransmit arrays may be superposed, thus allowing the use of a commontransmit/receive transducer.

FIG. 18(b) is a variation of the trapezoidal scheme in which the receiveand transmit system are lined up in series. In the context of a verticalcylinder with periodic boundary conditions, the transmit and receivearrays may be superposed and use a common transmit/receive transducer.

The rectangular sensor subsystem in FIG. 18(c) involves a 180°reflection off the bottom edge. The transmit and receive arrays may besuperposed, or indeed be identical, thus supporting the use of a commontransmit/receive transducer. As presented in FIG. 18(c), with reuse ofthe reflective array and transducer, such sensor subsystems are knownand employed by Carroll Touch in its ZOHPS products for X and Ycoordinate measurements.

In FIG. 18(d), the transmit and receive arrays scatter in directionsopposite to standard commercial rectangular sensors. This scheme isshown in FIG. 17 of U.S. Pat. No. 5,329,070 to Knowles. This schemeprovides additional opportunities for mode filtering of the scatteredacoustic waves and may be used to interpose time delays. Thus, asuperposed reflective array may be provided with, e.g., reflectiveelements disposed at ±45° to an incident acoustic beam, with onescattered path traveling opposite to the other. One of the pathsintersects a reflective boundary, which redirects it and/or modeconverts it in a desired manner. Further, this configuration may be usedto suppress certain parasitic paths. Finally note that the reflectiveboundary need not act as a mirror parallel to the reflective array, andtherefore may produce an arbitrary reflection angle.

Example 10

FIG. 19 illustrates a cylindrical sensor which includes sensorsubsystems of the types given in FIGS. 18(b) and 18(c).

For the purposes of this invention, the meaning of "planar surface" isextended to include any 2-dimensional surface of Euclidean geometry,i.e. surfaces that can be "unrolled and laid flat" without wrinkles orother distortions. A spherical surface is still non-planar; this is anobservation related to the map makers' problem for flat maps of theglobe. However, a cylindrical surface may be considered "planar"according to the present invention. A cylindrical sensor surface alongwith acoustic paths can be mapped to a flat surface with correspondingacoustic paths provided that the curvature is always large compared tothe acoustic wavelength; and in fact known touchscreens forcylindrically curved video monitors do not provide particularcompensation for the cylindrical shape. Hence the cylindrically curvedsensor of FIG. 19 may serve to illustrate application of FIG. 18.

The sensor in the form of a complete cylinder poses an interestingtopological twist to "planar" sensor geometry. The sensor of FIG. 19takes advantage of the periodic boundary conditions of a fullcylindrical surface 1901. As is apparent from inspection of FIG. 19,this eliminates the need for any vertical arrays.

The touch sensor of FIG. 19 includes the superposition of two sensorsubsystems 1902, 1903 of the type of FIG. 18(b) and two sensorsubsystems 1904, 1905 of the type of FIG. 18(c). There are tworeceive/transmit transducers 1906, 1907 each aimed at superposedreflector array 1908, 1909 containing three sets of reflectors.

With the aid of the principles of FIGS. 10 and 11, there are manyoptions regarding acoustic modes. The choice of acoustic modes willaffect the design of the reflective boundary at the top of the cylinder.If lowest order plate waves are used across the touch region, i.e.ZOHPS, lowest-order symmetric Lamb, or a flexural wave, the reflectiveboundary can simply be a machined edge as illustrated in FIG. 19(a). Forother types of surface acoustic waves, reflective-array type reflectiveboundaries may be used.

If there is not mode conversion at the reflective arrays, then thereflector angles are 45° for the sensor subsystems of the type of FIG.18(c), and the reflectors for the trapezoidal subsystems are at anglesof 45°±θ/2 where tan(θ)=πR/2H. If mode conversion is desired, then thespacing-vector formula may be used to calculate reflector angles andspacings.

Note that a burst from a transducer will generate three signals. Forsome delay times, all three received signals will be active. Severalapproaches can be used to disentangle such simultaneously receivedsignals.

Signals from the rectangular subsystems of the type of FIG. 18(c) areisolated. For these sigills, the receive transducer is the same as thetransmit transducer, in contrast to the four signals from thetrapezoidal subsystems of the type of FIG. 18(b). This is an example ofusing distinct receive transducers to separate signals generated by acommon transmit pulse.

In trapezoidal sensor subsystems, for a given choice of transmittransducer (the other choice simply reverses the directions of theacoustic paths), a burst generates two signals in the same time windowreceived by a common receive transducer. One approach here is to designthe reflector spacings of the two sensor subsystems to correspond to twodifferent operating frequencies. The frequency tuning of the receiverelectronics is then used to separate the signals; the transmit burst maybe broad band or a sequence to bursts at different frequencies.

Another approach is to use a phase-sensitive controller. The twosimultaneously received signals are allowed to combine with anuncontrolled phase at the receiver. However, regardless of the relativephase between the two received signals, a touch attenuation in onesignal will always be recognized by an amplitude change, or phasechange, or both, of the combined signal.

Signal separation with a phase sensitive controller may result in anambiguity in the identification of which perturbation in the combinedsignal corresponds to which component signal. See dotted ghost paths1910 shown in FIG. 19(b). For the sensor of FIG. 19, such ambiguitiescan be easily resolved with the aid of the coordinate data from therectangular sensor subsystems.

FIG. 19 is but one of many possible cylindrical sensor configurations.Other examples include systems with a touch surface on an inside surfaceof a cylinder; a single transducer with reflective array around fullcircumference; top and bottom arrays, side-by-side arrays with separatetransducers rather than superposed arrays; etc.

Example 11

FIG. 28 shows a planar sensor system which faces the same types ofambiguities as the cylindrical sensor according to FIG. 19. In thisembodiment, a single transducer 2801 and superposed reflective array2802 generates and receives sets of waves 2803, 2804 traveling at anangle to one another. In this system, as shown in FIG. 28(b2), a singletouch produces a pair of perturbations in a received signal. The averagetime delay for the pair of signal perturbations represents the distancealong the array from the transducer, while the separation represents thedistance from the bottom edge of the substrate. Note that a singlesensor subsystem observes the touch twice due to the forward andbackward wave paths. The embodiment of FIG. 28(a) includes a third setof wave paths 2805 which is orthogonal. In this case, one set of thereflective elements in the array 2802 is used to mode convert a Rayleighwave emitted from the transducer to a HOHPS wave at right angles.

The sensor of FIG. 28(a) provides redundancy that may supportanti-shadowing algorithms, multiple-touch algorithms, as well asdual-mode touch-characteristic algorithms.

FIGS. 28(b) and 28(c) further show that these three wavepaths may beused to determine the positions of a pair of touches. The method firstlooks for perturbations of the rectangular wave sets 2810. From these,the X coordinate is estimated 2811. Perturbations in the "W" shapedwavepaths are next determined 2812. An iterative loop 2813 is thencommenced to review all pairs of perturbations. The average delay iscalculated 2814. For each perturbation of the rectangular path 2815, theconsistency of the "W" shaped paths is determined 2816. Further, sincethe rectangular wave is a shear wave and the "W" wave is a Rayleighwave, the attenuation ratio may be determined to eliminate water dropletshadowing effects 2817. The confirmed touch position is then calculated2818, and output 2819. Further rectangular path perturbations are thenanalyzed 2820, and then the iterative loop 2813 incremented 2821.

Example 12

The concepts of FIG. 10 generalize to non-planar, i.e. non-Euclidean,surfaces. On a non-planar surface, acoustic waves naturally travel ongeodesics of the surface. For example, on a spherical surface, acousticwaves naturally follow great circles. The line segments of FIG. 10therefore generalize to the geodesics of FIG. 20.

All three segments of the acoustic path shown in FIG. 20 are geodesics.The path parameter "s" can be any parameter that is smoothly andmonotonically related to the arc length along the reflective arrays. Thedelay time as a function of the path parameter, t(s), can be determinedby the sum of the arc lengths of the segments, each divided by the groupvelocity of its acoustic mode.

A closed analytic expressions for t(s) typically does not exist for ageneral complex curved surface. Once the geometry of the sensor is knownor proposed, numerical calculations may be needed to calculate t(s) aswell as the wave vectors needed by the spacing-vector formula to computereflector angles and spacings as a function of s.

Reflector spacing and orientations are determined as a function of thearray and path parameter s by considering a small quasi-planarneighborhood in which the scattering takes place, and then applying thewave vector analysis in the same way it is applied to planar coordinatesubsystems. In this context, FIG. 11 represents a small quasi-flatregion of a general curved surface. The spacing-vector formula isequally applicable to planar and non-planar sensors.

As shown in FIG. 17, FIG. 10 can be generalized to cases in which theacoustic paths encounter reflective boundaries. While not explicitlyshown, the same is understood for FIG. 20. Reflective boundaries are noless applicable to non-planar surfaces as they are to planar surfaces,and in fact may advantageously be employed to increase options forsensor geometry or to simplify construction of the touch sensor.

As noted with FIG. 10, wave guiding effects can be used to designtransmit and receive arrays that cause the corresponding acoustic pathsto deviate from line segments. Likewise, FIG. 20 generalizes to cases inwhich the transmit and receive segments deviate from geodesics.

The principles of FIG. 20 enable numerous possible shapes for non-planarsensor design. A few examples follow. While there is pedagogic value toemphasize special cases which can be analyzed analytically, e.g.,sections of a sphere, the scope of the present invention is not subjectto such limitations.

Example 13

FIGS. 21(a) and (b) provides an example of a non-planar sensor. Here thetouch surface is a section of a sphere, qualitatively corresponding tothe Earth's surface between the equator and the arctic circle.

This sensor contains four sensor subsystems as follows.

A burst from transmit transducer T1 will be scattered at a 90° angle andproceed up a "line of longitude" until it is reflected by a reflectiveboundary near the top of the spherical cap, and retraces its path backto T1. This sensor subsystem covers a "twelve hour time zone". Similaracoustic paths starting and ending at transducer T2 provide a secondsensor subsystem which covers the other "twelve hour time zone".Together, these two sensor subsystems provide a φ coordinate measurementwhere T1 senses a touch if φ is negative and T2 senses a touch if φ ispositive.

Not shown in FIG. 21(a) are the "u" acoustic paths that start attransducer T1 and end at transducer R1. As shown in FIG. 21(b), at anangle u with respect to the T1/T2 transducer pair, the transmittedacoustic beam is scattered onto a great circle. This great circle istilted with respect to the vertical by an angle θ and passes between theR1/R2 transducers and the polar hole. When it intersects the equator, itis scattered onto the equator and completes its journey to R1.

Shown in FIG. 21(b) is an acoustic path of the "v" system which is verysimilar to the "u" sensor subsystem. This time the burst comes from T2and the signal is received at R2. The great-circle geodesic is againinclined at a tilt of angle θ, but this time goes between the T1/T2transducers and the polar hole and the associated reflective boundaryused by the φ sensor subsystems.

Neither the u or the v sensor subsystems cover the full touch region.However (neglecting possible dead regions due to the finite size oftransducers), all points on the touch surface are covered either by thecoordinate pair (u,φ) or by (v,φ). Two dimensional coordinates can besensed for the entire touch surface with the aid of an anti-shadowingalgorithm; here the shadows in question are due to the polar hole.

Looking at more detail at the sensor geometry, if any two of the threecoordinates, φ, u, v are measured, then the coordinates of the touch onthe spherical cap, (Θ,φ) can be determined. Here we define the angle ofthe touch with respect to the vertical direction, Θ, such that Θ≡0 onthe "north pole" and Θ=90°=π/2 at the "equator", where the arrays aresituated. Possible measured values of Θ are in the range θ<Θ<π/2.

    Θ=arctan(tan(θ)/sin(Δφ))=arccot(sin(Δφ).times.cot(θ)) where

    Δφ=(π-u-v)/2 for φ>0

    Δφ=(u+v-π)/2 for φ<0

or

    Δφ=φ-u for φ>0

    Δφ=|φ|+u-π for φ<0

or

    Δφ=π-φ-v for φ>0

    Δφ=v-|φ|for φ<0

If φ is not measured directly, it can be determined from u and v asfollows.

    If (u+v)<π, then φ>0 and φ=(u-v)/2+π/2

    If (u+v)>π, then φ<0 and φ=(u-v)/2-π/2

If φ and one of u and v is known, the other can be predicted as follows.

    u=2φ+v-π for φ>0 or u=2φ+v+πfor φ<0

    v=π+u-2φ for φ>0 or v=-π+u-2φ for φ<0

The angles u and v have ranges between 0 and π and are linearly relatedto delay times.

With the aid of the spacing-vector formula and the principles of FIG.20, there are many options for the choice of acoustic modes. As a simpleexample, consider the case in which only one acoustic mode is used forall acoustic path segments. In this case, the equatorial arrays willinclude three sets of superposed reflector elements as follows: 45°reflectors with nλ spacing; 45°-θ/2 reflectors with nλ(1-sin(θ))spacing; and 45°+θ/2 reflectors with nλ/(1+sin(θ)) spacing. Also, inthis case, the reflective boundary may be formed of a set of concentricrings with a surface spacing of λ/2.

There are many substrate options: glass, aluminum, enamel-on-aluminumLove-wave substrate, etc. The choice of acoustic mode and substrate donot affect the sensor geometry except in determining the maximum size.For example 5.53 MHz Rayleigh waves on aluminum can support sizes with adiagonal dimension larger than 30 centimeters.

In a particular embodiment, a hemispheric aluminum dome is providedapproximately 300 mm in diameter. Near the "equatorial" concave opening,a pair of reflective arrays are provided on the outer surface, eachextending almost halfway around the hemisphere and nearly meeting ateach side. On each end of each reflective array is provided anultrasonic transducer, such as a piezoelectric ceramic element, whichmay be mounted on a wedge, to generate or receive an acoustic wavetraveling parallel to the axis of the reflective array.

Near the apex of the dome, another reflective member is provided, havinga diameter of approximately 100 mm. This apical reflector may bestrongly reflective for perpendicular incident waves and weaklyreflective for possible parasitic acoustic paths at other angles ofincidence.

Each reflective array includes three sets of reflective elements:

1. 45° to axis of symmetry of hemisphere with spacing of nλ

2. 45°+θ/2 (35.5°) to the equatorial plane of the arrays with spacing ofnλ/(1-sin θ)

3. 45°-θ/2 (54.5°) to the equatorial plane of the arrays with spacing ofnλ/(1+sin θ)

Mirror reflection ambiguities in these reflection angles are resolved byconsidering the desired acoustic paths along with theangle-of-incidence-equals-angle-of-reflectance criteria. θ is the anglebetween the center of the hemisphere of the upper reflective member,about 26°.

In this case, portions of a wave transmitted from a first transduceralong one of the pair of arrays are emitted along great circles of thehemisphere, which are then received by the other reflective array,following a path parallel to the reflective array to a receivingtransducer. Other portions of the wave are directed directly toward theapical reflective member and back down toward the originating reflectivearray and originating transducer. Therefore, at any time, a wave emittedby one transducer is received by that same transducer as well as onereceiving transducer associated with the other array. Therefore, bothsystems may be operative simultaneously to receive acoustic waveswithout substantial mutual interference. The transmit transducers T1 andT2 may emit acoustic bursts sequentially.

Alternately, these arrays may be shingled, inclined at a small angle toallow the transducer of one array to sit distal to an end of the otherarray. Of course, the angles and spacing of the array must becompensated for this inclination per the spacing-vector formula. In apreferred embodiment, the transducers are tucked just below the equatorand the corresponding reflector arrays follow great circles that endpass just above the transducers at the opposite side.

Furthermore, there may be greater than two transducers and arrays,allowing a plurality of sensing waves to be emitted by a transducer andreceived by a plurality of transducers, according to the principles setforth herein.

It is noted that, on any substrate, and in particular in substrateswhich are nonplanar, the coordinate system employed to define theperturbation position need not be in a Cartesian or pseudo-Cartesiansystem, and therefore may be expressed in a polar coordinates, or inother terms. Further, in certain instances, extrinsic factors, such asan overlay or superimposed image may be used to define valid entries,and therefore the ambiguities in the input position are need notnecessarily be fully resolved solely through acoustic wave perturbationanalysis in every instance in order to provide a useful output.

Example 14

FIGS. 21(c) and 21(d) provide another example of a non-planar sensorutilizing the principles of FIG. 20. Again we consider a section of asphere. This time the touch surface corresponds to everything north ofthe "Tropic of Cancer", and the region between the equator and theTropic of Cancer is available for arrays and transducers. This system isdescribed in further detail below.

The dome sensor shown in FIGS. 21(c) and 21(d) is hemispherical inshape. The touch region is above the "Tropic of Cancer" at 23.5° Nlatitude and is redundantly covered with no dead regions by three pairsof sensor subsystems. Arrays and transducers are placed in the region ofthe hemisphere between its base or equator and the Tropic of Cancer. Thearrangement of transducers and arrays are shown in the flat-mapprojection given in FIG. 21(c). One of the six sensor subsystems isillustrated from a top view perspective in FIG. 21(d).

In the simplest embodiment with six sensor subsystems, each array arc inFIG. 21(c) corresponds to a single set of reflective elements forming anarray. However, according to the present invention, it is noted thatthese arrays may be superposed to support additional sensor subsystems,and therefore each transducer may be associated with a plurality of wavepaths, providing further redundancy. The simplest embodiment isdescribed in more detail below.

Equally spaced around the equator are six pairs of transducers, onetransmit and one receive. Each transducer pair supports one sensorsubsystem. Each of the following three sensor-subsystem pairs fullycover the touch region above the Tropic of Cancer: R1/T1 and R4/T4;R2/T2 and R5/T5; and R3/T3 and R6/T6.

For clarity of presentation only, FIG. 21(c) shows a gap between the endof the transmit array for T1 and the receive array for R4, and likewisefor other diametrically opposed transmit and receive transducers. Inpractice, it is desirable to extend both arrays so that there is anoverlap. This assures that there is no dead region, and in fact anoverlap between, for example, the R1/T1 sensor subsystem and the R4/T4sensor subsystem. The overlapping portions of the arrays have reflectorelements approximately mirror reflected with respect to the array axis.

Consider now in more detail an individual sensor subsystem, i.e., theR1/T1 sensor subsystem shown in FIG. 21(d), the transmit array follows asection of a great circle that intersects the X axis and is rotated byan angle Θ about the X axis with respect to the equatorial plane. Thetilt angle, say Θ=20°, is less than the 23.5° latitude of the Tropic ofCancer.

In analyzing this array, let R be the radius of the hemisphere. Then thetransmit array follows the following trajectory on the surface of thehemisphere.

    x(s)=R·cos(πs/2)

    y(s)=R·sin(Θ)·sin(πs/2)

    z(s)=R·cos(Θ)·sin(πs/2)

The definitions used here for the x, y, and z directions are shown inFIG. 21(d). Similarly, the trajectory for the receive array is asfollows.

    x(s)=R·cos(πs/2)

    y(s)=R·sin(Θ)·sin(πs/2)

    z(s)=-R·cos(Θ)·sin(πs/2)

In these formulae, s is the path parameter which nominally varies fromzero to one as the distances from the transducers increases. In thisexample, the array will start for a small positive value of s in orderto make room for the finite sized transducer, and the array will end ata value of s slightly greater than one in order to provide the overlapbetween the sensor subsystem pairs discussed above.

Now consider the (θ,φ) coordinate system for the surface of thehemisphere defined by the following relations.

    -π/2<θ<π/2

    0<θ<π

    x(θ,φ)=R·cos(θ)·cos(φ)

    y(θ,φ)=R·cos(θ)·sin(φ)

    z(θ,φ)=R·sin(θ)

In terms of this coordinate system, the transmit array follows thetrajectory:

    θ(s)=arcsin(cos(Θ)·sin(πs/2))

    φ(s)=arctan(sin(Θ)·tan(πs/2))

and the receive array follows the following trajectory:

    θ(s)=-arcsin(cos(Θ)·sin(πs/2))

    φ(s)=arctan(sin(Θ)·tan(πs/2))

The geodesic connecting the transmit and the receive arrays for the pathparameter s is a segment of a line of longitude with respect to the zaxis, namely the following section of a great circle.

    -arcsin(cos(Θ)·sin(πs/2))<θ<arcsin(cos(Θ).multidot.sin(πs/2))

    φ=arctan(sin(Θ)·tan(πs/2))

There are many options regarding choice of acoustic modes, and theparticular spherical configuration does not alter the general principlesof the invention. Let us consider in more detail the case in which thesame acoustic mode propagates along both the transmit and receive arrayswith group velocity V, while the mode, perhaps different, traversing thetouch region has the group velocity V'. The delay time as a function ofpath parameter is given as follows.

    T(s)=(R·(πs/2))/V+2R·arcsin(cos(Θ)·sin(πs/2))/V'+(R·(πs/2))/V

The delay time can also be expressed in terms of the coordinate φ of atouch which intercepts the acoustic path.

T(φ)=(2R/V)·arctan(tan(φ)/sin(Θ))+2R·arcsin(cos(Θ)·sin(arctan(tan(φ)/sin(Θ)))/V'With this analytic expression, a look-up table may be calculated. Such alook-up table can be used in real-time microprocessor code to convertmeasured delay times of signal perturbations into the touch coordinateφ.

More generally, while explicit mathematical analysis may be able todetermine a touch location on the surface, this analysis is notnecessary in some cases. Rather, the transducers produce a set ofoutputs for a given touch condition, e.g., a location. By empiricallydetermining a signature of this touch condition, the controller will beable to determine when this input condition subsequently occurs.Further, with a number of such conditions determined, an interpolationor statistical determination of the condition of an input determined,even if it does not identically correspond to a previously determinedinput condition. A lookup table is one way to store the data.Alternately, the data may be stored as coefficients of a compensationalgorithm for mapping the input space into a desired output space.

The transducer pairs R1/T1 and R4/T4 provide complete coverage of thetouch coordinate φ over the entire touch region.

Similarly R2/T2 and R5/T5 provide measurement of a touch coordinate uwhich is an equivalent to φ except that the polar axis, while still inthe x-z plane, is rotated 60° with respect to the z axis. Likewise R6/T6and R3/T3 provide a touch coordinate v which is an equivalent to φrotated -60°. The three coordinates φ, u, and v provide redundantcoverage of the dome sensor. In terms of x, y, and z coordinates, φ, u,and v are defined by the following relations. ##EQU1## The touchcoordinate θ can be determined from φ, u as follows. ##EQU2## Likewise,the touch coordinate θ can be determined from φ, v as follows. ##EQU3##

If θ(φ, u) and θ(φ, v) agree, then (φ, u, v) form a self consistenttriple of delay times; the meaning of self consistency of a triple isdiscussed above, e.g., in connection with item 2403 of FIG. 24(a). Thus,this sensor supports anti-shadowing and multiple-touch/redundancy-checkalgorithms.

If we define δθ=θ, δφ=φ-π/2, δu=u-π/2, and δv=v-π2, then the top domesensor corresponds to the values δθ=0, δφ=0, δu=0, and δv=0. Taylorexpanding the above relations about the top of the sensor gives thefollowing approximate relations. ##EQU4## Note the similarities for theplanar hexagonal sensor of FIG. 15(b) where Y is the coordinate measuredby the two sensor subsystems using transducers 1502, 1511, 1508 and1505, U is the coordinate measured by the two sensor subsystems usingtransducers 1504, 1501, 1510 and 1507, V is the coordinate measured bythe two sensor subsystems using transducers 1512, 1509, 1506 and 1503,and the c enter of the sensor corresponds to X=Y=U=V=0. ##EQU5## Thereis a quantitative analogy between X, Y, U, and V with δθ, δφ, δu, and δvin the small quasi-flat region at the top of the sensor of FIG. 21(d).In this sense, The sensor of FIG. 21(d) is a non-planar generalizationof the sensor of FIG. 15(b). Similarly, there are non-planargeneralizations of other planar sensors geometries.

Reflector spacing and angles can be calculated using previouslydiscussed principles. Let us refer again to this first sensor subsystemin FIG. 21(d). For the transmit array, the reflector spacing vector isS=2πn(k_(t) (s)-k_(p) (s))/|k_(t) (s)-k_(p) (s)|² where k_(t) (s) andk_(p) (s) can be calculated from the known array trajectory (θ(s), φ(s))given above by the following expressions.

    k.sub.t (s)=(2π/λ)·(-sin(πs/2), sin(Θ)·cos(πs/2), cos(Θ)·cos(πs/2))

    k.sub.p (s)=(2πλ')·(-cos(φ(s))sin(θ(s)), -sin(φ(s))sin(θ(s)), cos(θ(s))

Here λ represents the wavelength of the acoustic mode traveling alongthe transmit array and λ' represents the wavelength traversing the touchregion.

Note that the maximum acoustic path length for the sensor subsystems is(2π-2Θ)R. For Θ=20°, this becomes 5.585*R. For Rayleigh waves at 5.53MHz on aluminum or borosilicate glass substrates, this means the domesensor can have a radius in excess of 10 inches. Even larger sizes arcpossible if a lower operating frequency is used, or other means areprovided to reduce acoustic attenuation or tolerate weaker signalamplitudes.

Therefore, an application for the hemispheric dome sensor according tothis embodiment is, for example, in an interactive museum environment.For example, a 20 inch (or a 1/2 meter) diameter borosilicate-glass domesensor with a reverse-projection screen laminated on the back side maybe provided. Star patterns of the night sky, or a section of the Earth'sglobe may be projected onto the sensor. This system could support atable-top hands-on planetarium or an interactive globe exhibit. Thetouch surface, arrays, and transducers may be placed on the concave sideof the sensor; for example, an interactive touch sensitive aquariumportal may be provided, perhaps in combination with an ultrasonic fishfinding/identification system, in which the user points to sea creaturesthat may swim by. Many other applications can be imagined.

Example 15

FIGS. 22(a) and 22(b) provide an example which illustrates the inherentgeometric flexibility of the present invention. It shows a basin whichmay be thought of as a flattened and otherwise distorted hemisphere witha hole in it for a drain. Such a sensor geometry may be of interest as abasin perhaps containing a liquid. Furthermore the touch sensitivesurface is on the inside rather than the outside. The reflective arrays2201, 2202, 2203, 2204, 2205, 2206 are disposed with topologicalsimilarity to the hexagonal sensor of FIG. 15(b); there are sixsuperposed arrays, each with one transmit 2207, 2208, 2209, 2210, 2211,2212 and one receive transducer 2213, 2214, 2215, 2216, 2217, 2218. Aswith the hexagonal sensor of FIG. 15(b), a third set of reflectors maybe superposed on each array to support sensor subsystems involvingopposite pairs of arrays.

The sides of the basin 2200 are vertical at the locations of thetransducers and arrays. Thus the intersection of a horizontal plane atthe level of the transducers and arrays with the basin forms a geodesicloop. The acoustic paths along the arrays follow sections of this closedloop geodesic.

For the transmit and receive arrays of each sensor subsystem, we definethe path parameter "s" to be arc length of the path along the array fromthe transducer array divided by the total arc length of the array.

Conceptually, the geodesic paths across the touch region for value s canbe determined as follows. A string is anchored on the transmit arraycorresponding to the value of the path length parameter s. The string islooped over the convex surface of the basin 2200 so that it intersectsthe receive array at the location corresponding to the path lengthparameter s. The string is pulled taught, and the length of the stringbetween the arrays and the directions of the string at both arrays isobserved; this determines the path length of the geodesic across thetouch sensitive zone as well as the directions of the wave vectors ofthe geodesic where scattering takes place on the transmit and receivearrays. In practice, this conceptual scheme serves as the mathematicalbasis of a computer simulation algorithm that solves the acoustic pathgeometry. In this manner, all relevant geometric information of FIG. 20may be determined.

If the basin 2200 is very deep, e.g. not a flattened hemisphere butrather a stretched hemisphere, then the geodesics between the arraysmight not pass through the desired touch region. In the string analogy,pulling the string tight may cause the string to slip off the desiredtouch zone. In this case, the design engineer can either flatten thebasin geometry or introduce intermediate scatterings in the acousticpath with reflective boundaries.

For any choice of acoustic modes and substrate options, the principlesof FIG. 20 and the spacing-vector formula allow calculation of reflectorangles and spacings. As is typical of present commercial acoustictouchscreen design methods, modulation of array reflectivity (e.g., viareflector density, reflector height, or line width) can be determinediteratively by building prototypes, observing signal uniformity (or lackthereof), and improving the modulation of array reflectivity. Means arethus available to design reflector arrays for the sensor in FIGS. 22(a)and 22(b).

As an illustrative example, consider the following choice of acousticmodes and substrate. The basin 2200 is formed of aluminum with athickness of 1 mm which smoothly increases to a thickness of 3 mm withina centimeter of the arrays and transducers. The inside of the aluminumbasin is enamel coated, with an appropriate type and thickness of enamelto support Love waves at about 5 MHz, e.g. 100 microns of lead (or otherheavy metal) based enamel. For the acoustic paths across the touchregion, the lowest order Love wave is used. Along the reflective arrays,e.g., a third order symmetric Lamb-type wave as modified by the presenceof the enamel coating propagates. The array reflectors are formed asmodulations in an otherwise smooth inside surface of the aluminum basinand may be fabricated by milling, scribing, chemical etching,photoetching, photoresist, or stamping before application of the enamelcoating. The transducers (coupling to the Lamb-type waves) are wedgetransducers and are bonding to the outside or convex surface of thealuminum basin. Thus, both the transducers and reflective arrays areprotected from the environment in the basin 2200.

Note that for this particular choice of modes and substrate, the basin2200 can be partially or completely filled with water and still respondto and distinguish a touch due to a finger of sinilar perturbation thatprovides viscous damping at the touch surface.

From the perspective of perturbation analysis algorithm design, thedrain pipe hole 2219 shown in FIG. 22(b) maybe regarded as a generalized"contaminant". In this regard, note that the sensor design of FIGS.22(a) and 22(b) has sufficient redundancy to support anti-shadowingalgorithms.

After a use of the wash basin or toilet is detected, a water flow orflush is manually or automatically initiated. In this case, the acousticsensor may be used to determine when the bowl is emptied and cease waterflow when the contents are evacuated. In the case of a toilet, whichoperates according to a fixed cycle, a minimal cycle may bepreprogrammed, detecting when such a cycle is necessary, with repetitionas required to fully evacuate the bowl. Otherwise, a rate or duration ofwater flow may be modulated. Thus, a closed loop washing or flushingcycle control is possible.

Applications of acoustic sensors such as the above applications demandsensor designs of complex non-planar geometry as is enabled by thepresent invention.

Example 16

In some cases it may be advantageous to use the same spacing vector ofthe same reflective array for more than one sensor subsystem. Thisfurther extends the design options within the scope of this invention.

As an illustrative example, consider a reflective array along the Xdirection with a single set of uniformly spaced 45° reflectors on asoda-lime glass substrate. The array is illuminated by a transmittransducer which generates Rayleigh waves at either 5 MHz with or 2.5MHz. The Rayleigh wavelength is 0.025" at 5 MHz and 0.050" at 2.5 MHz.The spacing of the reflectors along the axis of the array is 0.100",that is, four Rayleigh wavelengths at 5 MHz and two Rayleigh wavelengthsat 2.5 MHz.

The spacing vector for this reflective array is S=(-0.050", 0.050").This spacing vcctol supports 90° scattering of Rayleigh waves at 5 MHz,90° scattering of Rayleigh waves at 2.5 MHz. and, as discussed below,for certain discrete thickness of the glass substrate, scattering of 5MHz Rayleigh waves at 71.56° into a plate wave.

The incident wave vector for Rayleigh waves is k_(I) =(2π/λ, 0) isevaluated as (251.3 inch⁻¹, 0) at 5 MHz and (125.7 inch⁻¹, 0) at 2.5MHz. The corresponding 90° reflected wave vectors k_(R) are given by (0,251.3 inch⁻¹) and (0, 125.7 inch⁻¹). In the spacing vector formula, thefactor 2π (k_(I) -k_(R))/|k_(I) -k_(R) |² is evaluated as (0.0125",-0.0125") at 5 MHz and (0.025", -0.025") at 2.5 MHz. The spacing vectorformula

    S=2πn (k.sub.I -k.sub.R)/|k.sub.I -k.sub.R |.sup.2

is satisfied with n=-4 at 5 MHz and n=-2 at 2.5 MHz. (Because reflectorspacing is a unidirectional concept, the algebraic signs of S and n arephysically irrelevant.)

Now consider the case of the incident Rayleigh wave at 5 MHz scatteredinto a plate wave at and a scattered plate wave 71.56°. Furthermore, thethickness of the substrate is adjusted to support a 5 MHz HOHPS platewave with a wavelength of 0.0316 inches. In this case, the reflectedwave vector may be calculated to be k_(R) =(62.85 inch⁻¹, 188.49 inch⁻¹)and the spacing-vector formula factor 2π (k_(I) -k_(R))/|k_(I) -k_(R) |²is evaluated as (0.0167 inch, -0.0167 inch). In this case, the spacingvector formula is satisfied with n=-3.

The substrate thickness "b" supporting a HOHPS wave of order "n" ofwavelength λ_(HOHPS) is given by ##EQU6## where at 5 MHz λ_(ZOHPS)=0.0267 inch. For example, at 5 MHz an n=4 HOHPS wave will have awavelength of 0.0316 inch if the glass substrate has a thickness of0.100 inches.

Signal equalization methods become more complex when the same spacingvector is used for more than one sensor subsystem; equalizing signalsfor one sensor subsystem may comprise signal uniformity for anothersensor subsystem. In this context, equalization techniques such asline-width modulation are of interest. Unlike varying the reflectorheight, varying the line width may differentially affect the scatteringamplitudes for different sets of waves. Note also that signalequalization techniques may be applied to any array or reflectiveboundary within a sensor subsystem.

This example illustrates a particular aspect of the invention, that thesame spacing vector S an array may support more than on set of wavesscattered.

Example 17

In this example we consider a feature of this invention from atouch-reconstruction algorithm perspective. This invention enablesredundancy checks. Redundancy checks are important to algorithms withmultiple-touch capability as well as being of value in their own rightfor rejection of false touches. Any sensor system in which a touch maybe sensed by three or more sensor subsystems may utilize algorithms withredundancy checks. Below we first consider general principles ofredundancy checks and then consider specific applications to specificsensor geometries.

FIG. 24(a) illustrates the basic blocks of a redundancy-check algorithm.This algorithm can either be used to provide enhanced rejection of falsetouches, resolve ambiguities due to multiple touches, resolveambiguities due to sensor geometry, or any combination of the above.

The identification of significant perturbations 2401 and determinationof delay times 2402 in FIG. 24(a) are implemented using techniquesdiscussed above. Existing commercial products use a variety ofsignatures to identify significant signal perturbations worthy offurther processing by touch recognition algorithm: sufficient amplitudechange; reproducibility of perturbation of signal with repeat burst;time duration characteristics consistent with finger touch; etc. Thetime delays for significant signal perturbations are also easilycalculated with a variety of averaging or centroid-finding methods toassign a unique time to perturbations of limited duration. Thus, thesesteps correspond to the low level signal processing and intermediatelevel perturbation extraction components of the control.

This scope of the present invention includes use of a phase sensitivecontroller and associated signal processing equipment to implement thislow level signal processing. With appropriate controller electronics,signal processing algorithms can utilize perturbations in phase as wellas amplitude to detect and interpret perturbations of the signal; seediscussion of associated with FIG. 19. In some cases, use of a phasesensitive controller enables sensor designs in which signals from morethan one sensor subsystem are simultaneously received by a singlereceive transducer and ambiguities exist as to which perturbation isassociated with which sensor subsystem; in these cases redundancy checkalgorithms may be used to resolve these ambiguities.

The third box of FIG. 24(a), the identification of self-consistenttriples 2403, is specific to sensors that can sense a touches with threeor more sensor subsystems. (More generally, as illustrated by the sensorin FIG. 28(a), it is sufficient to have three independent delay timeseven if provided by less than three sensor subsystems.) Here thealgorithm identifies self-consistent triples of delay times, e.g.,perturbations of all three received signals may be consistentlyinterpreted, within measurement errors, as corresponding to a commonlocation on the two-dimensional touch surface. There are many choicesfor the algorithm details for testing for self-consistency. One optionis to convert all delay times into coordinates and then check theconsistency of the coordinates. Alternatively, two delay times may beused to calculate two coordinates and hence a touch position which inturn can be used to predict the third delay time; prediction andmeasurement for the third delay time are compared. In some cases, It ispossible to check self-consistency of delay times without everconverting delay times to coordinate values. Other options exist. Theessential ingredient is a test for self-consistency of three or moredelay times with a single touch position on the touch surface.

The fourth box in FIG. 24(a) represents the processing of the identifiedself-consistent triples into a list of zero, one, or more touchcoordinates in the desired coordinate system 2404. Severalself-consistent triples may correspond to a single touch; this istypical when a touch is sensed by four or more sensor subsystems. Thedesired coordinate system for output of touch positions need not bedirectly related to the natural coordinate systems of the sensorsubsystems. For example, in the spherical cap sensor of FIGS. 21(a) and21(b), the (Θ,φ) coordinate system may be used to output coordinatesdespite the fact that Θ is not measured directly by any of the φ, u, orv sensor subsystems.

It is possible to combine the third and fourth boxes in various ways.For example, in a sensor for which a desired touch region is reliablysensed by four sensor subsystems, one may test candidate self-consistentquadruples of delay times. A self-consistent quadruple contains fourself-consistent triples of delay times. While the block diagram of FIG.24(a) emphasizes the minimal requirement of three measurements for aredundancy check for touches on a two-dimensional surface; it isimplicitly understood that algorithms with the same net effect may beemployed.

For a sensor designed to measure only one coordinate of a touch, then aredundancy algorithm may be possible with self-consistent doubles ofdelay times. For example, a sensor containing a Rayleigh-wave X sensorsubsystem and a shear-wave X sensor subsystem, a redundancy-check may bebased on a requirement that both sensor subsystems measure the samevalue of X within errors.

Now consider the X-Y-U sensor in FIG. 13 in more detail. FIG. 23presents schematic signals from the sensor system according to FIG. 13,having a soda-lime substrate, propagating 5.53 MHz Rayleigh waves, and atouch at a location indicated in FIG. 13, namely 5.75 inches up and 5.75inches across from the corner of the touch region having no transducers.Approximate delay times are given in FIG. 23 for this particular case.

The X-transmit-to-X-receive signal provides an X coordinate measurement.The Y-transmit-to-Y receive provides a Y coordinate measurement. TheX-transmit-to-Y-receive signal and the Y-transmit-to-X-receive signalprovide a U coordinate measurement. U is related to X and Y as follows.

    U.tbd.-sin(Θ))X+cos(Θ)Y

The diagonal angle is Θ=30°, or more generally the inverse tangent ofthe sensor's aspect ratio.

Delay times, t, for each sensor subsystem are linearly related to X, Y,or U. If the corner of the active region with no transducers is definedas the origin, then the coordinates are related to delay times asfollows.

    X=W+H/2-Vt/2

    Y=H+W/2-Vt/2

    U=-Wsin(Θ)·(W+W/cos(Θ)-Vt)/(W+W/cos(Θ)-H) for X-to-Y-receive

    U=+Wsin(Θ)·(H+W/cos(Θ)-Vt)/(H+W/cos(Θ)-W) for Y-to-X-receive

V is the group velocity for the acoustic mode used. For the touchposition indicated in FIG. 13, the X coordinate is reconstructed fromthe perturbation at 205 microseconds in the X sensor subsystem, the Ycoordinate is reconstructed from the perturbation at 155 microseconds inthe Y sensor subsystem, and the U coordinate is reconstructed from theperturbation at 175 microsecond in the Y-to-X-receive sensor subsystemusing the second of the U expressions above.

In practice, the constants in the equation above will typically bedetermined from a calibration procedure in order to properly account forvarious electronic offsets and path length effects due to the finitewidth of reflector arrays. The real value of formulas like those aboveis to provide a means to check that empirically derived linear mappingcoefficients "make sense".

The 205, 155, and 175 microsecond delay times in FIG. 23 form aself-consistent triple for the 30° X-Y-U sensor of FIG. 13. This may beverified by reconstructing the X, Y, and U coordinates from the delaytimes, and then testing the resulting values with the followinginequality.

    |U-sin (Θ))X-cos(Θ))Y|<ε

where ε is the measurement tolerance and Θ=30°. The tolerance ε isadjusted to be large enough to accommodate reasonable measurement errorsand uncertainties and yet small enough to reject combinations of delaytimes derived from more than one touch. Its numerical value depends onmany factors. In many cases the optimal value of ε will be in the rangefrom 0.1 inches to 0.5 inches.

Using the linear relations between X, Y, and U to delay times, theinequality above may be re-expressed directly in terms of delay times.It is not necessary to convert delay times to coordinates to test forself-consistency of triples of delay times.

Another variation of the self-consistency check is to determine twocoordinates, say X and Y, which may then be used to determine a point onthe touch surface (X,Y), which in turn is used to predict a delay timefor the third sensor system, say U.

Self-consistent triples may be used to resolve ambiguities due tomultiple touches. For example, consider the case in which the sensor inFIG. 13 is subjected to two touches, on at location (X, Y) and the otherat (x, y). Data from the X and Y signals can be interpreted as a pair oftouches at (X,y) and (x,Y) as well as a pair of touches at (X,Y) and(x,y). However, only the (X,Y) and (x,y) interpretation will lead toself-consistent triples of delay times when X, Y, and U data iscombined.

If the sensor of FIG. 13 is subjected to three simultaneous touches atlocations (X,Y), (x,y), and (X',Y'), then there are six ways tointerpret the X and Y signals: [(X,Y), (X',Y'), (x,y)]; [(X,Y), (x,Y'),(X',y)]; [(X,y), (x,Y'), (X',Y)]; [(X,y), (X',Y'), (x,Y)]; [(X,Y'),(x,Y), (X',y)]; [(X,Y'), (X',Y), (x,y)]. Again, using U coordinate datato identify self-consistent triples of delay times will uniquelyidentify the three touch positions. In principle, the same principleapplies to any number of simultaneous touches. In practice, the finiteresolution effects and overlapping of signal perturbations will resultin practical limits in the number of simultaneous touches that can besorted out. In any case, either instead or in addition to knowntechniques, the present invention enhances multiple touch capability.

Multiple touches may lead to cases in which one touch "shadows" anothertouch. For example, consider two simultaneous touches in the sensor ofFIG. 13 in which the two touches have similar X coordinates butdifferent Y and U coordinates. For best multiple-touch performance, thepreferred embodiment is to use algorithms that incorporate theanti-shadowing algorithms along with the redundancy check methodspresented in this example.

If four or more sensor subsystems sense a touch, then it is possible formore than one self-consistent triple of delay times to be associatedwith a touch. For example, with the X-Y-U-V sensor of FIG. 14, a touchmay result in four self-consistent triples involving any of thefollowing combinations of coordinates: (X, Y, U); (X, Y, V); (X, U, V);(Y, U, V). Alternate self-consistent triples for the same touch can beassociated because they all lead to the same reconstructedtwo-dimensional touch location.

Without the φ sensor subsystems, of the sensor of FIG. 19 would still beable to reconstruct touch positions with the u and v sensor subsystems.However, such a reduced version of the sensor in FIG. 19 would besubject to ambiguities due to sensor geometry; see "ghost" touchindicated in FIG. 19B. This ambiguity is resolved with the addition ofthe φ sensor subsystems and the use of self-consistent triples.

Let us look at the sensor of FIG. 19 in more detail. From the delaytimes, one can determine the values of the coordinates u, v, and φ whichare defined to be the angle in radians about the cylindrical axis fromφ=0 to the point on the transmit array where the acoustic waves arescattered. Touch positions can be reconstructed from the u and v delaytimes. The cylindrical coordinates (φ,z) are determined by u and v bythe following relations.

    φ=(u+v)/2 mod 2π

    z=(H/πR)×{|[(π+u-v) mod 2π]-|}

Now consider the case were the u and v signals are simultaneouslyreceived and disentangled with a phase-sensitive controller. If thephase-sensitive controller does not have means to identify whichperturbation belongs with which of the simultaneously received signals,the acoustic paths labeled u' and v' provide an alternate interpretationof the signal data, which in turn leads to the reconstruction of theghost touch location indicated in FIG. 19(b). This is an example of adiscrete touch location ambiguity. However, note that (u,v,φ) will forma self-consistent triple while (u',v',φ) will not. The ambiguity isresolved.

Example 18

A touchscreen with multiple touch capability may be utilized as a 3-Dinput device.

For example, consider a transparent cylindrical sensor enclosing a threedimensional display having a rotating helical sheet within the cylinderupon which pixels are projected. In this case, two coordinate axes, hand φ, are easily obtained and interpreted from a touch. However, itwould be desirable to be able to acquire touch informationrepresentative of the third axis, ρ, the radial distance from the axisof the devices. With the coordinates from two simultaneous touches, (h₁,φ₁) and (h₂, φ₂), there are many choices for a mapping to the threedimensional space (h, φ, ρ), e.g., (h, φ, ρ)=([h₁ +h₂ ]/2, [φ₁ +φ₂ ]/2,R|φ₂ -φ₁ |) where R is the radius of the cylinder. With three touches,there is enough information (six parameters) to control both theposition and orientation of the image of a solid body in threedimensional space.

In a particular embodiment of a cylindrical sensor for such anapplication, a borosilicate glass cylinder is provided having a heightwhich equals its circumference, e.g., 15 inches. At a lower portion ofthe cylinder, three sets of side-by-side arrays are each provided, eacharray having an associated transducer. A first reflective array hasreflectors at 22.5°, a second reflective array has reflectors at 45°,and a third reflective array has reflectors at 67.5°. Thus, threeacoustic waves are generated, two of which having paths which are ±45°with respect to a central axis of the cylinder, with other having anaxis parallel to the central axis of the cylinder. On the top of thecylinder, a single transmit transducer emits a pulse on a superposedarray having three sets of reflectors. In this case, a four transducersystem is implemented without any reflective arrays on a side portion.This system may be used, for example, as a touch interface for threedimensional displays having a rotating helical sheet within the cylinderupon which pixels are projected.

It is noted that these dual or multiple touch analyses are a result ofthe availability of ambiguity resolving data and a motivation foranalysis thereof, and are not limited to cylindrical touchscreensystems. Therefore, systems which are flat, spherical sections, or ofother geometry may be used as three or greater dimensional inputs.

Alternatively, a third axis may be obtained by analyzing an intensity ofa touch, a so-called Z-axis. This Z-axis, however, has low resolutionand may be difficult to control. Thus multiple-touch capability providesa more precise and reliable 3-D input means.

Example 19

The feasibility of anti-shadowing algorithms is an outstanding featureof sensor designs in which at least portions of the touch regions issensed by three or more sensor subsystems. This enables enhanced sensorperformance in the presence of contaminants as well as addingflexibility to sensor design options.

FIG. 25(a) is a generic flow chart for an anti-shadowing algorithm. Inknown sensors, loss of a coordinate measurement due to shadowing resultsin a failure or inability to reconstruct the two-dimensional position ofthe touch. Here shadowing refers to the disruption of the acousticsignal passing through the touch position due to contaminants, anothertouch, or obstacles intrinsic in the sensor design, at a location distalfrom the touch.

The first two boxes 2501, 2502 in FIG. 25 are the same as for FIG. 24and are discussed above in that connection. They will not be discussedfurther here.

The third box refers to the pairing up of delay times corresponding tosensor subsystems with overlapping touch regions 2503. The fourth boxcalculates the positions of touch 2504. The basic feature of theanti-shadowing algorithm is that good data from two sensor subsystems issufficient to reconstruct the position of a touch on a two-dimensionalsurface. It is not necessary for all sensor subsystems to makesuccessful measurements. More generally if a touch is within theacceptance of N sensor subsystems, then an anti-shadowing algorithm may"pair up" 2, 3, . . . , or N-1 delay times.

Anti-shadowing algorithms use data from a third sensor subsystem toimprove touch-detection efficiency. This contrasts with theredundancy-check algorithm in which data from a third sensor subsystemis used to improve false-touch rejection. If a touch is sensed by fouror more sensor subsystems, then anti-shadowing and redundancy-checkfeatures are easily combined in the same algorithm. If only three sensorsubsystems are available, more complex algorithms may be used thatconditionally utilize anti-shadowing and redundancy-check features.

Now let us consider anti-shadowing algorithms in the context of specificsensor geometries.

Consider the X-Y-U sensor in FIG. 13 and the associated signals of FIG.23. In this case, three pairings can be made, each of which allows thedetermination of the (X,Y) coordinates of the touch. Referring now toFIG. 23, the 205 microsecond X delay and the 155 microsecond Y delayallows determination of (X,Y) as is known. However, the 205 microsecondX delay can also be combined with the 175 microsecond Y-to-X delay. Inthis case the missing Y coordinate can be determined by the equation##EQU7## where V is the acoustic group velocity (assuming all modes thesame) and t is the Y-to-X delay time. Similarly, if the 155 microsecondY delay is combined with the 175 microsecond Y-to-X delay, the missing Xcoordinate can be determined by the following equation. ##EQU8## If thetouch is in the zone covered by the X-to-Y sensor subsystem, then thecorresponding equations are as follows. ##EQU9##

The X-to-Y and the Y-to-X sensor subsystems are an example of sensorsubsystems that do not have overlapping touch zones. The algorithm inFIG. 25 does not attempt to pair up delay times from such pairs ofsensor subsystems.

In many cases the redundancy-check algorithm of FIG. 24(a) and theanti-shadowing algorithm of FIG. 25(a) can he combined. For example,consider the X-Y-U-V sensor oFt FIG. 14, in which a touch is typicallysensed by four sensor subsystems. One coordinate measurement can be lostdue to shadowing, and yet three coordinate measurements will remain tosupport an algorithm requiring a self-consistent triple of delay times.

The cylindrical sensor of FIG. 19 also provides an application for thistype of analysis. As is evident from inspection of FIG. 19(b), any orthe following three coordinate pairs, (u,v). (u,φ), and (v,φ), issufficient to determine the (r, φ) coordinates of the touch.

The spherical cap sensors of FIGS. 21(a) and 21(b) and FIGS. 21(c) and21(d) provide other examples. In these cases, any of the three possiblecoordinate pairs (u, v), (u, φ), and (v, φ) is sufficient to determinethe (Θ, φ) coordinates.

For the sensor of FIGS. 21(a) and 21(b), the anti-shadowing algorithm isessential to assure two-dimensional touch reconstruction for the entiretouch surface. The v sensor subsystem has a blind region to between thehole and the transducers R1 and R2; in this region (Θ,φ) coordinates arereconstructed from the (u,φ) sensor subsystem pair. Similarly the usensor has a blind region between the hole and the transducers T1 andT1; in this region (Θ,φ) coordinates are reconstructed from the (v,φ)sensor subsystem pair.

Similarly, the anti-shadowing algorithm of FIG. 25(a) can be used tooptimize touch performance of polygonal sensors such as the hexagonalsensor of FIG. 15(b) and large sensors such as the large rectangularsensor of FIG. 16(a).

In general, the redundancy-check algorithm of FIG. 24(a) and theanti-shadowing algorithm of FIG. 25(a) allow one to make beneficial useof sensor designs employing redundant coordinate measurements.

Example 20

If a touch is sensed with more than one acoustic mode, then anadditional characteristic beyond touch position and "Z axis" touchpressure may be determined. Such information may be used, for example,to reject false touches due to water drops on the touch surface.

FIG. 26 outlines the basic parts of a dual-mode touch characteristicrejection algorithm. The first box, delay times from different sensorsubsystems for a touch are associated as a byproduct of thetouch-position reconstruction algorithms 2601; more generally, the firstbox represents the group of delay times from different sensors thatcorrespond to a single touch regardless of whether the touch location isactually computed. The second box represents the determination of themagnitude of the signal perturbations for the delay times associatedwith the touch 2602; it may be that the magnitudes of the signalperturbations have already been calculated as part of a test ofperturbation significance. Here it is assumed that not all signalperturbations involve the same acoustic mode in the touch region. In thethird box the signal perturbations are compared with expectedcharacteristics, e.g. ratios of perturbation amplitudes, of validtouches 2603.

FIG. 26 illustrates the basic features of a dual-mode algorithm. Inpractice, the dual-mode feature may be incorporated in various ways intoalgorithms that reconstruct touch positions, perhaps determine touchpressure, perhaps provide anti-shadowing and multiple features, etc. Theessential feature here is the comparison with expectations of therelative magnitudes of coupling of two or more acoustic modes to atouch.

As an example, consider the sensor shown in FIG. 14 for an embodiment inwhich the X and Y sensor subsystems sense touches with a horizontallypolarized shear wave (ZOHPS HOHPS, or Love), and in which the U and Vsensor subsystems sense touches with an acoustic mode with verticalparticle motion at the surface, such as Rayleigh and Lamb waves. To bemore specific one may, for example. use a 0.090 inch thick soda-limesubstrate at an operating frequency of 5.53 MHz for which the Rayleighwavelength is 0.0226 inches where the U and V reflector angles andspacings are given in FIG. 13(b) and FIG. 14 and the X and Y reflectorspacings are integer multiples of the Rayleigh wavelength and the X andY reflector angles are about 52.5° as needed to couple Rayleigh waves ton=4 HOHPS waves traversing the touch region.

If such a sensor is subjected simultaneously to a water drop and afinger touch, due to viscosity damping, the finger touch will result inexpected amplitudes of signal perturbations in both the (X,Y) and (U,V)subsystems. However due to the weak coupling of horizontally polarizedshear waves to water, the (X,Y) signal perturbations due to the waterdrop will be weak while the (U,V) signal perturbations will be strong.The weak (X, Y) signal for the water drop will not be interpreted as alight finger touch because the corresponding (U,V) touch is strong. Theratios of signal perturbations for the same touch thus provides acharacteristic of a touch that differentiates between water drops andfinger touches. With empirically determined thresholds for such ratios,the algorithm can respond to finger touches and yet reject touches fromwater drops.

The algorithm of FIG. 26 has other uses besides water rejection. Forexample, such an algorithm can be used to verify that a user is properlywearing gloves provided that the type of glove is constructed of amaterial that has a ratio of radiation-damping to viscosity dampingcharacteristics that is sufficiently distinct from bare finger touches.This feature could be used, for example, to assure compliance withsafety procedures for equipment where the wearing of gloves ismandatory.

Example 21

A test reflective array is provided having continuously varyingreflector angles, from 45° to 56° with respect to the axis of the array.Other ranges of reflector angles may also be of interest. This arrayserves to produce, at various portions of the substrate, increasingreflector angles that may be experimentally tested for mode-conversionscattering at 90° of an incident Rayleigh wave to a plurality ofpropagation modes. A useful feature of 90° scattering is that thereflector spacing along the axis of the array depends only the inincident mode and not the reflected mode. The reflective array acts as adiffraction grating, directing waves having varying phase velocities atdifferent positions along the arrays.

It has been found that the optimal chevron angle from the axis ofpropagation of an incident Rayleigh wave for scattering at right angles,for a shear wave of n=0 is about 46°, n=1 is about 47-48°, n=2 is about48°, n=3 is about 50°, n=4 is about 52-53°, and n=5 is about 56°, forglass thickness of 0.085" to 0.090", with increasing thickness tendingto smaller angles. The ratio of the phase velocity of a Raleigh wave andan n=4 HOHPS is about 0.92.

Example 22

The output of a dual-mode algorithm as represented by FIG. 26 need notbe limited to a simple pass/reject judgment on the nature of the touch.The dual-mode algorithm may categorize valid touches amongst a discreteset of categories, or even provide an analog measure of a touchcharacteristic.

"Dual-mode" algorithms need not be limited to the use of only twodistinct acoustic modes. Use of three or more acoustic modes is alsowithin the scope of this invention. In this context "distinct acousticmodes" may refer to the same acoustic mode at a significantly differentfrequencies, e.g. Rayleigh waves at 2 and 5 MHz. The essential featureis that not all sensor subsystems couple to a touch in the same way.

A dual-mode algorithm with a discrete-set output has application withsensor systems used with multiple styli. For example, a set of styli maybe provided in which each stylus has a tip with a unique acousticcoupling properties. The unique acoustic coupling properties may be, forexample, a particular ratio of coupling strength to Rayleigh-waves viadominant leaky-wave damping mechanism to the coupling strengthhorizontal-shear motion via viscous damping. When a user draws on thetouch surface with a stylus, the dual-mode algorithm enablesdetermination of the particular stylus used. For example, in anelectronic white-board application, different styli may correspond todifferent colors; depending on whether the electronic white-board iscombined with a display technology, the styli may or might not alsodouble as markers applying physical (in contrast to "electronic") ink tothe touch surface.

The dual-mode algorithm may be combined with other techniques to furthercategorize the nature of the touch. For example, the time duration ofthe touch perturbation may be used to help distinguish different stylivia the size of the contact area between the sensor and the stylus tip,as is considered in claim 10 of European Patent Application 9411927.7.The stylus tip may be vibrated at a signature frequency, e.g. 100 Hz, inorder to modulate the magnitude of the touch perturbation is a fashionthat can be recognized by a controller algorithm. These and othermethods may be combined with the dual-mode algorithm to more reliabilityor more completely characterize the nature of a touch perturbation.

Here "stylus" generalizes to anything that results in a touch. Forexample, consider an acoustic sensor per this invention built into thebottom of a drip pan. More particular imagine that liquid drops aresensed by both ZOHPS and flexural (lowest-order anti-symmetric Lamb)waves. The ratio of ZOHPS to flexural perturbation magnitudes is largerfor a high viscosity oil drop than a low viscosity gasoline drop.

As an example of an analog output of a dual-mode algorithm, consideragain the above drip-pan application. The ratio of ZOHPS signalperturbation, a measure of viscosity, to the flexural wave perturbation,a measure of leaky-wave attenuation which is weakly dependent onviscosity, is a measure of viscosity. Hence with a dual-mode algorithm,this invention supports viscosity measurement. It is known thatblood-count is strongly correlated with blood viscosity, so a "drip pan"blood-count sensor may provide a portable sensor with fast response; inthis case the sensor substrate may be a glass slide and the operatingfrequency may be above 5 MHz to reduce size and increase resolution.

In a blood drop viscosity measurement system, the reflective arrays maybe formed as a screened frit on the glass slide or as an etched orground structure. However, where the slide is disposable, thetransducers may be provided separately and as a part of a permanentfixture. Thus, the transducers are pressed tightly against the glassduring testing to couple the acoustic waves, without a permanentadhesive bond.

Example 23

As shown in FIGS. 32(a)(1) and 32(b)(2), an adaptive thresholddetermination scheme may be implemented with regional variations tooptimize the sensitivity of the touchscreen without causing undueerrors. This adaptive threshold scheme has two slightly differentaspects. First, during initialization, the system rapidly acquiressufficient data to allow perturbation detection. Then, afterinitialization, the threshold is adaptively updated, excluding portionsof the sensor for which significant perturbations are detected.

Thus, the system initially seeks to determine a baseline input 3201,presumably in the absence of touch, for each available subsystem. Duringinitialization, the system may also detect and ignore significanttransient perturbations which may be due, for example, to prematuretouches, and thus the processing scheme for the first and second aspectsof the adaptive baseline processing may be merged. The baselinecharacteristics are stored 3202. It is noted that this baselinecharacteristic data is generally stored separately for each sensorsubsystem of the device. Based on the stored baseline characteristicsfor each sensor subsystem, a statistical analysis of the normalvariations, instability and noise may be made, which may provide a basisfor setting a margin between the normal baseline and a threshold 3203.The threshold may vary based on a signal space of the sensor subsystem,based on the baseline stability in a given region of the sensor or timedelay after transducer excitation 3204. In a normal operational mode,the baseline is determined 3206, without reference to detectedperturbations 3205, and adaptively updated 3207. In addition, thebaseline stability characteristics 3208 and threshold 3209, which mayeach vary based on a position or region of the sensor, and for eachavailable sensor subsystem, are also adaptively updated.

According to the present invention, a single emitted acoustic wave maygive rise to a plurality of received signals, representing differenttransducer subsystems. Therefore, as shown in FIG. 32(b), a receivedsignal may be analyzed for resolution of information relating to aplurality of sensor subsystems 3211, 3213, 3215. The system willgenerally sequentially measure the signals from each available sensorsubsystem 3210. However, in some cases, available redundancy may allowthe sensor to operate in the absence of data from one or more sensorsubsystems. Further, at any given point in time, sufficient data may beavailable for certain analyses, even though a complete mapping of thesensor for each subsystem is not complete.

If the received signal is above the threshold for a given position andsubsystem 3212, which, for example in a phase sensitive receiverembodiment, is evaluated 3216 as √((ΔI)² +(ΔQ)²)--Threshold (position,subsystem)>0, further analysis ensues 3217. Otherwise, no perturbationis deemed detected 3218, and the system continues to receive and analyzefurther data, e.g., from the next sensor subsystem 3220. On the otherhand, if the data from a sensor subsystem is superthreshold, asignificant perturbation is detected, and this information passed 3219to higher level, baseline analysis, or other algorithms.

As shown in FIG. 32(c), after data for some or all available sensorsubsystems is obtained from the perturbation detection algorithm 3221, adetermination may be made whether sufficient data is available toproceed with analysis 3222, which may differ for the various algorithms.Further analysis of the perturbations according to the present inventionmay then be performed, as appropriate, including an anti-shadowalgorithm 3223, a multiple touchiredundancy algorithm 3224, and aconsistency algorithm 3225. Normally, the size and shape of aperturbation will also be analyzed 3226, to allow an optimal outputcoordinate to be calculated. If sufficient perturbation data is receivedand analyzed 3227, which as stated above need not include all the data,or data from each sensor subsystem, then a further process ensures thata coordinate representation of the perturbation(s) are normalized into adesired output coordinate space 3228. The actual normalization orcoordinate transformation may be performed at various points in theprocess, and indeed various portions of the process may operate indifferent spatial representations of the perturbation position(s). Thenormalized coordinate representations are then output 3229. Theperturbation analysis is a continuous process, analyzing eachsignificant perturbation. Thus, the processes set forth in FIGS.32(a)(1), 32(a)(2), 32(b) and 32(c) may proceed independently andasynchronously, except where data from one process is required foroperation of the other process.

The present invention therefore extends the field of acoustictouchscreens by describing such systems which innovate the mechanicalconstruction, receiver electronics and or logical processing systems,for the purpose of providing, among other advantages, increasedflexibility in packaging and configuration, improved performance, andthe ability to process multiple perturbations simultaneously. It shouldbe understood that the preferred embodiments and examples describedherein are for illustrative purposes only and are not to be construed aslimiting the scope of the present invention, which is properlydelineated only in the appended claims.

What is claimed is:
 1. A substrate for an acoustic touch sensor systemcomprising:an acoustically transmissive medium having a surface with atouch sensitive region having at least one side; at least two reflectivearrays, disposed parallel to one another and on the same side of thetouch sensitive region, together having a two dimensional Fouriertransform with at least two useful spacing vector components.
 2. Thesubstrate according to claim 1, wherein said two useful spacing vectorcomponents are for waves scattering at different angles.
 3. Thesubstrate according to claim 1, wherein said two useful spacing vectorcomponents are for waves of differing frequencies.
 4. The substrateaccording to claim 1, wherein said two useful spacing vector componentsinclude at least one acoustic wave mode conversion.
 5. The substrateaccording to claim 1, wherein said reflective arrays are superposed. 6.An acoustic sensor comprising:an acoustically transmissive medium,having at least one surface adapted for sensing proximity of an object;at least two sets of acoustically reflective elements formed on saidmedium, each set of elements having at least two elements and forming arespective reflective array having at least one useful spacing vectorcomponent of its two-dimensional Fourier transform, a spacing vector ofa first reflective array being non-orthogonal and non-identical to aspacing vector of a second reflective array.
 7. The sensor according toclaim 6, wherein said two useful spacing vector components are for wavesscattering at different angles.
 8. The sensor according to claim 6,wherein said two useful spacing vector components are for waves ofdiffering frequencies.
 9. The sensor according to claim 6, wherein saidtwo useful spacing vector components include at least one acoustic wavemode conversion.
 10. The sensor according to claim 6, wherein saidreflective arrays are superposed.
 11. The sensor according to claim 6,wherein said reflective arrays are not disposed coaxial or perpendicularrelative to each other.
 12. The sensor according to claim 6, whereinsaid medium comprises glass.
 13. The sensor according to claim 6,wherein said reflective elements comprise raised acoustically reflectivematerial formed on a surface of said medium.
 14. The sensor according toclaim 6, further comprising an electronic control adapted to receive anddistinguish acoustic waves corresponding to a spacing vector of saidfirst reflective array and a spacing vector of said second reflectivearray to detect a characteristic of the object.
 15. The sensor accordingto claim 6, further comprising an electronic control system forreceiving acoustic waves corresponding to a spacing vector of said firstreflective array and a spacing vector of said second reflective array,said electronic control system analyzing said received acoustic waves inacoustic wave phase sensitive manner.
 16. The sensor according to claim6, further comprising an electronic control adapted to receive anddistinguish acoustic waves corresponding to a spacing vector of saidfirst reflective array being different from a spacing vector of saidsecond reflective array to detect the object.
 17. A touch sensorcomprising:an acoustic wave transmissive medium having a surface, atleast a portion of said surface being adapted for sensing a proximity ofan object; and an array, associated with said medium, having a pluralityof spaced elements for scattering portions of an incident acoustic waveas waves each having a different propagation vector than said incidentacoustic wave, and passing other portions unscattered, said array beingsituated along a path, said array having associated propagation vectorsof scattered portions of the acoustic wave which are not substantiallyperpendicular to the incident acoustic wave over the array, said pathbeing distinct from a segment parallel and adjacent to a side of arectangular region of a small solid angle section of a sphere.
 18. Amethod of forming an acoustic sensor comprising:providing anacoustically transmissive medium having at least one surface adapted forsensing proximity of an object; forming on the medium an array, having aplurality of spaced elements for scattering portions of an incidentacoustic wave as waves each having a different propagation vector thansaid incident acoustic wave, and passing other portions unscattered,said array being situated along a path, said array having associatedpropagation vectors of scattered portions of the acoustic wave which arenot substantially perpendicular to the incident acoustic wave over thearray, said path being distinct from a segment parallel and adjacent toa side of a rectangular region of a small solid angle section of asphere.
 19. The method according to claim 14, wherein said forming stepcomprises depositing acoustically reflective material on a surface ofthe medium.